Emperor Calus appointed a younger Ghaul to Primus of the Red Legion, where his talents would be best put to use instead of a Gladiator. For his albinism, Ghaul became known as the Ghost Primus. Under his command, the Red Legion became loyal to Ghaul over their emperor, such that when Ghaul launched his coup against Emperor Calus, the Red Legion accompanied him, arresting Calus and forcing him aboard The Leviathan.
The Legion was last deployed in response to the Skyburners' distress call to attack the Last City on Earth, a significant force launched a successful invasion of Earth and the Last City while attacking other places like the Reef. Using the Traveler Cage, they managed to cripple the Last City and devastate the Guardians, leaving them vulnerable and displaced. They are attempting to claim the Light from the Traveler and become Guardians.
Following the attack, the Red Legion began to quickly occupy the Solar System everywhere, even making an attempt to dig up ancient Vex ruins on Nessus, where they unearthed the ancient Vex mind Protheon, Modular Mind. On Io, they attempted to drain Io of its abnormal light which accidentally summoned the Taken, forcing them to leave their many dig sites. On Mars the remaining Legions gathered for one final assault and the Red Legion led the charge. The stalemate was broken, the Vex Virgo Prohibition was destroyed, Bastion retaken, and the Gate to the Black Garden was disabled. On Mercury the Titan Sunbreakers were destroyed.
At some point, the former Emperor Calus has received news of Ghaul’s forces in the Sol System, believing it was the right time, he sent his Shadows to eliminate the Dominus once and for all, however, despite their best efforts, all of them have died horribly. 
With their security being lax, and Ghaul being preoccupied by wanting to understand the Traveler, their greatest weapon, The Almighty, was disabled by a Guardian. This led to a full on attack on the Red Legion-occupied City by more Guardians, spearheaded by the Vanguard. With pressure mounting and his temper boiling over, Ghaul murdered the Consul, who criticized his obsession, and went to take the Light to become a Guardian himself. For a short while he fought the Guardian. Ghaul was defeated and the Traveler awoken, who ended the Dominus once and for all. Even in it’s aftermath, Red Legion weapons are used in Shaxx’s Crucible to help train new Guardians.
Ghaul's death and the Legion's defeat at the City did not substantially hinder their war machine. With the Dominus's command succeeded by Val Ca'uor, the Red Legion began to actively occupy Mercury and Mars in order to resupply their forces. Red Legion forces headed by Kargen, the Technocrat hacked the Insight Terminus on Nessus for Vex technology while Psions were also attempting to evolve into Flayers by tapping into the Warmind Rasputin on Mars; both efforts were thwarted by the Guardians.
Around the time the Hive faction, the Grasp of Nokris, began emerging from the Hellas Basin on Mars, Val Ca'uor led a Red Legion fleet against the Leviathan in order to seize it from Calus and weaponize it. Ca'uor's forces were repelled by The Guardian and was assassinated afterward, thus leaving the Red Legion leaderless again, weakened and further demoralized.
Events after the Warmind
However, this was not the end of the Red Legion, the Guardians started attacking the Legion more and more aggressively, instigating a skirmish with the House of Dusk Fallen, shutting down their anti-air defenses at Firebase Hades, allowing air bombing runs and raids to take place on the facility, directing the Taken assault on the EDZ toward the Legion, taking their fuel and supplies, and with the discovery of lost sectors, their leaders to came out of hiding, allowing multiple assassinations to take place. On Nessus, their commander in The Conflux, Primus Cal'aug, was killed by The Young Wolf, stopping the Legion from undergoing any further operations on Nessus. On top of that, Firebase Hades was disabled, most of their supplies were stolen and they had to deal with heavy losses due to failed offensives against the City and The Fallen.
Meanwhile on Mars, things were supposedly going well for the Legion and was a sign the Legion was still a major force with it being the only place the Legion still had power over. With the Ice melting after impacts from the surface, a new threat emerged, The Grasp Of Nokris and with that they were now seeing there progress on Mars quickly being undone in Hellas Basin they now fought a war with an enemy that they didn't understand, it is unknown if they were fighting the Grasp of Nokris all over Mars or it was isolated to Hellas Basin.
In addition to their disheveled state, the Red Legion began suffering from multiple desertions such as those following Garut Gra'am who rooted themselves in Trappers Cave. The most prominent of these groups, the Unbowed Legion, made their debut in the EAZ during the Solstice of Heroes event in an attempt to take over the EAZ as a base of operations. Despite the group being led by three large Cabal commanders similar to Ghalak the Colossus, Forge Warden, they were quickly put to an end by the Guardians partaking in the event.
At some point during Forsaken, Red Legion could be found in the Tangled Shore as well as a group of Cabal have found their way into the Cathedral of Scars in the Dreaming City during Gambit matches.
More recently a family group of Psion Flayers, having commandeered Osiris' time-traveling Sundial, split Mercury into three alternate timelines that threaten to span beyond the planet and into the system. It is their intention to undo Ghaul's defeat, or, failing that, gather enough Light from the future to buy their way back into Calus' favor. Though the Psion Flayer sisters, Niruul, the Hollow Voice, Ozletc, the Sky Piercer and Tazaroc, the Sun Eater, managed to create a border realm for themselves, the Guardians would ruin their plans. One by one, the Psion sisters would face the Guardians and be defeated. After weeks of their forces being eliminated, the three sisters, driven to desperation, used their vast psionic powers to merge into a single being, Inotam, Oblivion's Triune. Despite Inotam's vast powers, the Guardians would nonetheless prevail against her, thwarting the sisters efforts to subvert time. However, one sister remains, Amtec, who was informed by her sisters of their possible demise and now schemes to avenge them.
After their crippling defeat at the Sundial, the Red Legion, headed by the escaped Psion Flayer, Amtec, decide to use the Almighty to crash into the Last City itself. With little to no choice, the Vanguard decide to cooperate with the Warmind Rasputin, with Guardians arming his arsenal, in order to save their home from destruction.
Despite Amtec's efforts, however, the Almighty was destroyed by Rasputin, with the help of the Guardians, before it could collide with Earth, resulting in Amtec's plot ending in a total failure for the Red Legion. Amtec's final fate is unknown, but she was possibly onboard when the Almighty was annihilated and perished in the explosion.
With the Almighty's crash site no longer in flame and its debris no longer falling from the sky, the Red Legion were reduced to a barren few after countless losses. Despite this, the Legion still operates. Several groups of Cabal interfere with the Contact public event in hopes of harnessing the phantasmal energy seen utilised in many Champions brought to the forefront. It is unknown if the Red Legion in their desperation have managed to use the energy to create Champions, or if the champions are derived from another way. It is also unknown whether or not Amtec still leads the remains of the Legion due to her possible death on the Almighty unconfirmed. As for their current fate, the remaining Cabal in the system will either turn to Calus’ Loyalists or to Caiatl, the Princess-Imperial’s regime, with the latter seeking to raise the Legion once more.
Named Leaders and Bosses
The command structure and tactics of the Red Legion are more aggressive and agile than any Cabal Legion observed before, with deadlier weapons, advanced technology, and battle techniques. The Red Legion also has a handful of new units that operate in the field. Majors will typically wear silver armor, whereas Ultras wear gold armor.
List of appearance
Red Legion troops can be primarily identified by their signature gunmetal and red color scheme, with some troops having gold, silver, or orange highlights depending upon their rank. They are also more heavily armored than other Cabal Legions, wearing metal plates over a bodysuit of War Beast scales. 
List of appearances