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Emperor Calus is the former Emperor of the Cabal Empire, who was usurped by Dominus Ghaul prior to the Red War. Following the demise of Ghaul, Calus travelled to the Sol System aboard his Leviathan to recruit the Guardians as his Shadows to have by his side as he prepared for the coming of the Black Fleet and what he believed to be the end of all things.
Having communed successfully with The Witness, the exiled Cabal became Emperor Calus, Herald of the Witness; joining forces with the Witness and the Black Fleet as one of their newest Disciples. His consciousness having fully integrated with the Darkness through the Egregore fungi that infests his now derelict Leviathan, Calus sought control over the Lunar Pyramid by psychically melding his mind fully with the vessel. Instead, the former emperor was challenged by a coalition between the Vanguard and his daughter Empress Caiatl, to which they would thwart his connection with the Pyramid.
Many years prior to the invasion of the Solar system, Calus led a populist revolution to eject the military aristocracy known as the Praetorate from power and became Emperor. By his side during the revolution stood his long-time friend and protector Shayotet, who stopped numerous attempts to kill Calus. Under his rule, the Cabal Empire became a seat of all-encompassing hedonism and revelry. His accomplishments during that time included redistributing Praetorate wealth to the people, granting citizenship to the Imperial Legions, and putting down various alien threats such as the Sindu, the Clipse, and the Psions' OXA Machine. However, when the OXA machine was rebuilt by the savant Otzot, Calus was impressed enough by her genius to order the machine preserved and hold a festival in her honor, elevating Otzot to his court as Imperial Dreamer.
His reign was not without personal troubles. Calus was wary of Umun'arath, his Evocate-General, for her paranoia and obsessive fixation on war. His own daughter and heir, Caiatl, grew up similarly despite his efforts. 
One of Calus' pleasures was the Coliseum, and one day, an albino gladiator, bleached from prolonged exposure to solar radiation due to being cast out as an infant, named Ghaul made his first appearance there. Ghaul's patient skill and unusual coloration soon made him a favorite of Calus', and the Emperor eventually made him Primus of the Red Legion, where he believed his talents would be put to better use as the head of the most fearsome Cabal legion ever assembled. Calus grew to love Ghaul like a son, and believed they were kindred spirits who both envisioned a newer, more perfect world. Ghaul's association with The Consul and members of the Praetorate worried him, but not enough to cause him to take any action, with the former having great animosity for the Emperor, for he was stripped of rank and title, humiliated and castrated by Calus during the Emperor's coronation, and told him to run into the paved wasteland.
Coup and Exile
While Calus and his Loyalist Cabal were engaged in pompous revelry, Ghaul launched his coup in the dead of night. With the Red Legion storming into Calus’ palace, seizing and/or killing political members, the emperor immediately ran to his throne to retrieve an Ahamkara bone he meant to save him from this fate. Caiatl, however, was waiting for him sitting upon it with the bone in hand. Despite his pleas for help, his daughter crushed the bone in hand, and declared she would not be weak. With the Emperor too popular to simply execute, Ghaul, his Red Legion, and the Consul arrested Calus and an unknown number of supporters in his own court and exiled them aboard The Leviathan, imprisoned within the Menagerie and taken far from the Empire along a pre-programmed course, far into the cosmos.
On the Leviathan, Calus wallowed in despair for an unknown amount of time until a strange event shook him out of his stupor. The Leviathan had come up against an expanse of utter nothingness, at the very edge of the universe, shorting out the navigation systems and the course the conspirators had set. Calus looked into the void and came to an understanding, believing that it had chosen him to herald its inevitable victory. He would watch it consume the light of the universe, and all among the last of the living would know his greatness.
With control of the Leviathan back in his hands, Calus set out on a long voyage of his own, "[plumbing] the secret places of the cosmos", visiting strange realms and old ruins. This cemented both his conviction that his hedonistic philosophy was correct and his intent to retake his empire. Along the way, he gathered forces to strengthen his Loyalists. He created the Shadows as his elite champions, allied with aliens such as the Clipse and the Sindu, and sent messages promising that whoever slew his conspirators would be given great wealth and glory. Eventually, he set a course for the Sol system, which the Red Legion was planning to assault. He sent his Shadows on a mission to assassinate Ghaul which failed, but he learned of other warriors in the system that might serve his purposes.  
For an unknown period of time, Calus has been using the Leviathan to consume planets and grind their soil into dust to mix with spores from the flowers in the Pleasure Gardens to create "Royal Wine", which he would then consume to give himself vast psionic powers.
Return to Power
After the death of Ghaul, Calus immediately set to work on reclaiming the empire. He gave elements of the now leaderless Red Legion the chance to "join the loyalist regime," sending them on missions to collect geological data on Nessus with the intent of converting the soil of the planet into Royal Wine. The Leviathan arrived at Nessus soon afterwards and began consuming it.
Calus extended an invitation to The Guardian to board his vessel for slaying Ghaul. The Guardian accept Calus' challenge and lead a raid team through Calus' palace and defeating his loyalist army; confronting Calus in person in his throne room. Seemingly facing off against the exiled Cabal Emperor, the Guardians witness his strange powers and after dealing critical damage, they find that the Calus they are facing is actually a robotic Automaton. Regardless, the Guardians succeed in destroying the robot. Before the machine's destruction, Calus boasts about his power and claims to know the "real truth" about the galaxy. Nonetheless, he rewards the Guardians for completing his challenge and offers them a place by his side should they seek him out. As the Guardians step past the machine's body into a large room to claim their reward, they see countless robotic bodies, all in the image of Emperor Calus.
At some point months later, the Leviathan consumed a chunk of Nessus that contained a powerful Vex Mind, Argos, Planetary Core, the entity responsible for converting Nessus into a perfect Vex world, which then caused the world-eater to clog up and malfunction. Emperor Calus immediately called upon the aid of the Guardians to destroy the Vex intrusion, where they succeeded and repaired the Leviathan. However, as they destroyed the Axis Mind, the barrier protecting itself was removed, thereby threatening the Guardians to be consumed by the fiery inferno of the Leviathan. Calus then saved the Guardians from being sucked into the Leviathan afterward, where he rewarded and congratulated them for their efforts. The Cabal Emperor once again offers them more than just the power of the Light, as Calus demonstrated that he can keep them safe. He also warns them that the "end" is near and therefore must be ready, as he will find them again.
When Val Ca'uor and the Red Legion invaded the Leviathan directly, Calus called upon the Guardians once more to expel them. Several of his Loyalists were slain and one of his duplicates destroyed personally by Ca'uor; however, with Calus assisting the Guardians with his own psychic powers, Ca'uor was killed, the Red Legion Fleet decimated and his forces routed. Calus watches with glee as Ca'uor dies and is impressed that the Guardians have grown "fat from strength and power." Calus tells the Guardians that he awaits the day they will stand by his side when the end comes.
Season of Opulence
At one point, Calus contacts the Guardian via Werner 99-40 and gifts them the Chalice of Opulence, a symbol of their nomination as a Shadow of Calus. He also invites them to the Menagerie to compete for pleasure and glory.
Around the time, Calus bred Gahlran as a Shadow of the Hive in order to wear the Crown of Sorrow. This plan goes awry as Gahlran is corrupted by the Crown's powers and goes insane, much to the emperor's chagrin - though it was ultimately fortunate that he chose another to bear its power, as the Crown was in reality an attempt by Savathûn, the Witch-Queen to put him under her control. Nonetheless, with Gahlran under the control of the Witch-Queen, the mad Shadow rallied the imprisoned Hive within the depths of the Leviathan's Menagerie. Wanting his former Shadow put down, Calus calls upon his Guardian allies once again, offering more riches and rewards upon their success. After a perilous struggle against the Sorrow-bearer and the Hive's wicked rituals, the Guardians succeeded in putting Gahlran down and retrieving the crown, thereby saving the Leviathan from the threat of a Hive incursion.
Pleased with their success, Calus rewards the Guardians as promised but also officially makes them his Shadows of Earth, creating a Tribute Hall in honor of their deeds. Several of the Guardians would accept Calus' offer and pledge themselves up as Shadows, much to the suspicions of Aunor Mahal of the Praxic Order. Despite the mistrust in the Cabal Emperor though, Ikora Rey and Zavala agreed to leave Calus alone citing his deterrence against the Red Legion, though under continued Vanguard surveillance.
Later on, still wanting a Shadow to command the Hive, Calus sends an invitation to Eris Morn, a former Guardian Hunter who has a deep history with the Hive. Knowing of such history, the exiled emperor aims to have Eris Morn become the Shadow of the Hive in place of Gahlran.
Arrival of the Pyramids
A year later, Calus would relay word that emissaries of his daughter Caiatl were tracing his movements, having murdered a Shadow agent of his in the process of eliminating Red Legion personnel with The Fourth Horseman.
With the appearance of the Black Fleet and the disappearance of planets in the Sol System, Calus and his Loyalists had gone into hiding along with the Leviathan having left Nessus' orbit. Soon after Caiatl began rallying the Cabal remnants across the system to her side, calling for the capture of her father.
Finding the Chosen
It was later revealed that Calus, in conjunction with a small crew of Loyalists and a Guardian confidant named Katabasis, attempted to communicate with the Darkness using Scorn aboard a ship called the Glykon Volatus to demand an explanation for its actions, which Calus had seen as underwhelming. Further investigation would reveal that Calus also desired to obtain power from the Darkness using the same manner as the aforementioned communion, and share it with the crew he had sent. The experiments conducted went awry, however, and the Glykon was soon overrun with mysterious growths that killed all non-Scorn inhabitants onboard and transformed one of the Ravagers into a powerful entity dubbed the Locus of Communion. The ship would also be subjected to geometrical anomalies and be plunged into extradimensional space due to the Darkness's influence.
Months later, the ship would reappear and broadcast a distress signal to the Cabal in the European Dead Zone, with Commander Zavala under the understanding that Caiatl's forces would potentially follow it. The Guardian would use the situation as an opportunity to infiltrate the Glykon, destroy the Locus and its Scorn, and retrieve the Dead Man's Tale from Katabasis's deformed corpse.
Calus's fate following the experiments was unknown. Although the Locus's ability to copy his voice suggested he may have somehow perished in the fallout, the Locus was said to have spoken to Calus in his own voice even before he vanished.
What is currently known however is that Calus appears to be funding the Psion insurgency from his Leviathan. During the Season of the Risen, Qabix, a member of the enclave, began to broadcast Psionic propaganda that exhorted the members of the Imperial Cabal to mutiny and attempted to entreat them to defect. As Caiatl could not launch an attack on the broadcasting station without being viewed negatively in the army's eyes, she entreated the Vanguard to step in. A fireteam was deployed which managed to slay Qabix and shutdown the broadcasts, despite the threat of an orbital bombardment being constantly present. Upon reaching the broadcast room, the fireteam discovered a chest and several other pieces of finery that appeared to have come from the Leviathan, and viewing of the propaganda revealed it to be a message involving a Psion, a golden statue of Calus offering "Salvation", and a Pyramid.
Ascension to Herald
After disappearing for two years, Calus returned with the Leviathan as a Disciple of The Witness, aiming to create a bond between his flagship and the Pyramid on the Moon. He now has reinforced his Loyalist troops with Scorn, as well as also controlling Nightmares created by the connection with the Pyramid. The Guardian learns from Caiatl that when the Leviathan appeared in orbit of the Moon, two hundred Loyalists departed and fled. The Loyalists encountered in the ship itself are newly spawned clones who act as an extension of Calus's will.
As Eris Morn and the Vanguard began Operation Midas, The Guardian finds destroyed robotic copies of Calus and speaks with Calus through them. During an attempt to sever the Nightmare of Uldren Sov from the Crow, Calus tells The Guardian that they are just a grub fattened on the lies of the universe. He also tells them that he too was like them and that he will enjoy seeing The Guardian's reality shattered. He would further be amused with the Guardians efforts to sever his connections to the Pyramid, getting into heated conversations with their allies in the process.
Calus would ultimately succeed in his plan to become a Herald of the Witness and enter the Lunar Pyramid after manipulating his daughter, Caiatl, into brazenly challenging the Nightmare of Dominus Ghaul, leading to the severance ritual's failure, which gave him the time he needed to merge with the Pyramid. However, as the transformation had not fully taken affect, Eris gathers the Guardian, Crow, Zavala and Caiatl to conduct one last large scale severance ritual to stop Calus from fully taking control of the Lunar Pyramid. Though Zavala and Crow would be attacked by Calus's Loyalists, the Guardian and Caiatl would find themselves within the Lunar Pyramid itself, meeting at the Chantry of the Darkest Hour. There Caiatl angrily demanded her father to show himself, in which her father did so by revealing his true psionic form. The father stated his disappointment in his daughters failures, doubts and of her decision to not stand by his side. Unleashing Fading Nightmares of those the Guardian had battled against along with a horde of Scorn and Loyalists, the Guardians were able to wound Calus by striking down the Fading Nightmares and having the Nightmare Harvester absorb them. After the last Nightmare had been defeated, Calus had been pushed back, disappearing in a psionic blast. Caiatl would confirm to Zavala that her father is gone, having given himself over to The Witness.
Though Calus had been defeated and the Leviathan is now silent, Eris claims that the battle was not a victory, as Calus had succeeded in his goal of becoming a Herald of the Witness, thereby becoming the harbinger of a Second Collapse. Thus, Eris states they had merely bought themselves time and now they must prepare themselves for when The Witness returns.
Personality and traits
Calus' personality is defined by five key characteristics: his hedonism, his narcissism, his ruthlessness, his need to be adored, and his utter certainty in the ultimate victory of the Darkness. As a hedonist, he loves opulence, extravagant displays, and pleasures of all kinds. As a narcissist, he considers himself godlike in status and power, is incapable of accepting any point of view other than his and immerses himself in royal scenery that often depicts his own image. When roused or denied, he is fully willing to use force to achieve his goals, throwing the old Praetorate to the mercy of mob justice without remorse and annihilating the Sindu fleet when they refused to join his loyalists. Fundamentally, however, Calus wants to love and be loved by those he rules. He abolished the Cabal Praetorate as he believed they abused their control of the Cabal Legions for their own benefit at the expense of the people's welfare, attempted to free the indentured Psions but was overthrown before he could complete the act, and considered the Consul and Ghaul turning the Cabal into a warmongering empire to be the ultimate betrayal to their way of life. 
Altogether, Calus is a figure of contradictions. He is an emperor enthroned by and supporting populism; he is capable of genuine altruism immediately following ruthless conquest; he is self-centered but dotes on his subjects; he is sure of the end of all things and his own death, but he is spurred to action and preserving what he can.
Calus has been imbued with potent psionic abilities by his Psion council, having the ability to pull individuals into what is called a mind prison; composed out of the sheer force of his psionic energy similar to an Ascendant Realm, making him a force to be reckoned with. Calus also claims to have knowledge on the true origins and purpose of the Traveler, and to have encountered a far greater power during his travels.
Following his encounter with the "black edge," Calus had apparently undergone profound physical changes. His Councilor, Match, claims to have seen him ingest exotic and dangerous substances, such as liquid helium-4 and pure neutronium, which would instantly kill any normal biological being. Some time after these events, Calus ceased to appear in the flesh, instead making exclusive use of his robotic proxies when addressing his advisors. It is implied that his robots no longer bear any resemblance to him as he is now, as in his own words, "I was not what I appeared to be" and "...my form and my strength are inextricably linked." 
His interest in the Guardians continues to grow, as he hopes they will become his Shadows and even saved them from being destroyed by the Leviathan, as they both defeated Ghaul and proved themselves to him that there was more to them than just the powers they possess. Calus laments the brutal, militaristic nature of the Cabal Empire as it is now, and desires Guardians to lead efforts to restore it to its former glory.
Calus' decision to embrace his hedonism partly stems from the fact that he knows how the end of the galaxy is approaching quickly, and how he wished to revel in each small delight until the end comes. He states that the reason he won't kill the Guardian is because he's enamored with a warrior who won't yield in the face of such overwhelming despair. Overall, the only goal Calus strives for is the right to be the last survivor before his own end at the upcoming apocalypse.
Calus appears to take issue with the Queen of the Reef, though not the Awoken people in general. His scribes state that at some future date his Shadows will kill the Queen for her defiance, and he personally mentions that if the Guardians come to live on the Leviathan that they cannot return to the Dreaming City. This likely has to do with their conflicting crowns, as Calus would not accept subservience to anyone but himself and the Queen notably bows to no one. He also loathes and pities the Hive, viewing the sword-logic as something they use as an excuse to hide their own fear of death; he states that given their repeated defeats at the hands of the Guardians, they would kill themselves as soon as they encountered them and save the Guardians the hassle if they truly believed it. Nonetheless, he wishes to civilize the Hive and induct them into his empire, saving them from themselves.
Emperor Calus will initially be immune to damage and will attack from afar with his Emperor's Gaze laser. He will summon an onslaught of Loyalists that must be killed, after which four unique Psions will appear on each of the four plates around the throne room. They each will each have a unique symbol above them: a War Beast, a chalice, a sun, or a pair of axes like the ones carried by Gladiators. These symbols need to be memorized for the next section. After a brief pause, Calus will clap his hands twice to teleport all Guardians into his mindscape, where they will be met with a giant projection of his head. Killing the Psions beforehand will just spawn extra enemies.
There will be three Psionic charges that will teleport three players back to the throne room, while the remaining three are stuck inside. Those who remain inside will be inflicted with the Mind's Eye debuff, to show that they are being sucked in. If they are sucked in too far, then the Guardians will die. Those who remain in the throne room will have to kill more enemies, and the Psions from earlier will appear with the symbols appearing above them. In the mindscape, Calus' projection will show a symbol on his forehead. Each of the three players will each see a different symbol. Whichever symbol has not been seen is the symbol whose Psion in the throne room needs to be killed. This will repeat a total of four times for each symbol that does not appear. Those being sucked in, meanwhile, with have to kill two different kinds of Psions: regular Psions and their projections. The regular Psions must be killed so that their ruptures cannot send players into the air, and the Psion projections must be killed so that Calus doesn't use his Mental Collapse attack. As soon as they are all killed, Calus' projection will unleash a Psionic Overload, where he will spew purple skulls from his mouth. This is a sign that Calus is charging up an Imperial Nova attack that can wipe the entire team. He will be shielded while doing so. Destroy the shield to stop the attack, while the people inside will shoot down the skulls. Killing or damaging a skull grants a stack of the "Force of Will" buff, and these stacks are shared by the whole team. The more stacks the team carries, the more damage they can deal to Calus. After a while, Calus' projection will stop spewing Psionic Overload skulls, and three Psionic charges will appear that will teleport each player back into the throne room.
And thus, the damage phase begins. All players must step on one of the four plates to deal damage. Since there are four plates, there are four chances to get in as much damage as possible. However, Calus will charge an Afterburn blast that can shut down the plate and kill everyone standing on it, thus causing players to jump to a new one after the timer for that plate has expired. After losing a set portion of health, parts of his body will break off, revealing a machine. He will also gain new attacks in addition to his laser: an arm-mounted Vengeance Cannon and a meteor storm that leaves behind circles of fire which deal damage over time. As his health reaches approximately 33%, he will charge up his Imperial Nova attack again. If Calus is not killed by the time the last plate stops giving the "Force of Will" buff, the team must start the encounter again, this time with some holes that appear in the mindscape, as well as different shields around the Incendiors and Centurions that appear in the throne room.
Prestige Mode Changes
During the mindscape phase, whoever kills the Psion whose symbol was not called out will be teleported to the mindscape, and one person will be teleported into the throne room to replace them.
Challenge Mode Changes
All four plates must be used at the same time during the damage phase.
List of appearances