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The Light, also known as the Gardener or the Sky is a paracausal force associated with the Traveler. Its counterpart is the Darkness, which it has been at war with since before the beginning of time.
Like the Darkness, the Light is a paracausal force that has existed since before the beginning of time and space. It is fundamentally driven to increase the complexity and diversity of entities within the universe, and in particular to encourage the growth of new lifeforms.
The Light emanates from the Traveler, but it is not exclusively produced by it. All living things and all places possess some Light—possibly as a soul—as the spark of life is Light in itself. Ulan-Tan hypothesized that all Light is connected, later confirmed when Guardians discovered the Hive siphoning light from the Traveler through a piece of itself. The Light is antipathic to the Hive, though they are known to feed on it and offer it as a sacrifice to their gods.
Guardians are the rare few able to wield the Light as a weapon. Additionally, the Light was used by the Traveler to construct the Ghosts. Orbs of Light may also be found, which give power to a Guardian's Super, their ultimate use of the Light. Light, as used by the Guardians, comes in three different varieties; these are the elemental damage modifiers, and are used in both a Guardian's chosen subclass and in certain weapons. It is possible for Light to be corrupted, as the shard of the Traveler within the European Dead Zone is corrupting an area of the surrounding forest. In addition, the Light Dominus Ghaul wielded was corrupted due to him taking Light from the Traveler, which was an act of Darkness.
Like the Darkness, the Light appears to be intelligent in some sense. However, whereas the Darkness seems all too willing to communicate with its servants in their own languages, the Light appears to be more aloof and cryptic, communicating in symbolic visions and signs, if at all.
According to the Lore Book "Unveiling," which is seemingly narrated by the Darkness itself, both the Light and the Darkness have existed since before the start of time, and thus before the universe came into existence. Being unbound by the laws of causality, neither the Light or Darkness can be said to have a beginning. Rather, they are emergent properties of even more fundamental "mathematical structures" that in turn underlay reality itself. 
In the allegorical story recounted in the Lore Book, the Darkness and Light occupied themselves by engaging one another in a game, analogous to a vastly more complex version of the "Game of Life" devised by Earth mathematician John Conway. This game would consistently arrive at an end-state dominated by a single, self-perpetuating pattern, which subsumed all others in the game. The Darkness found this to be a pleasing outcome, but the Light felt the pattern to be boring, and desired that the game endlessly produce novel patterns instead. In order to promote this novelty, the Light transformed itself into a new "rule" within the game; the Darkness did the same, to counteract the Light's efforts. These new rules were "set aside" from the other rules of the game, so that they could not be limited or influenced by them, but could manipulate the other rules to bring about their intended outcomes.
While the story above is laden with metaphor and open to interpretation, it appears to suggest that the universe is the "game" which the Light and Darkness played in the garden, that the rules of the game are the laws of physics as they are commonly understood, and that the "new rules" that the Light and Darkness became are the basis of paracausality.
The Ancient Past
The earliest recorded mention of the Light comes from the Books of Sorrow, a collection of ancient Hive texts chronicling the origin of the Hive and their history of interstellar conquest. The Light is referred to as "The Sky," and is described as a philosophy that upholds collaboration and gentleness with the goal of creating a thriving, harmonious society and universe. Its counterpart, "The Deep," is also described, being a philosophy claiming that existence is the struggle to exist, and it is the goal of all life to challenge and defeat other life so as to become stronger and continue existing. Several disciples of the Light, including the Leviathan and the Traveler, also feature in these texts.
Before the rise of the Hive, the Traveler had visited several civilizations and bestowed the blessings of the Light. These civilizations included the Ammonite and the Harmony. At an unknown time in the past, the Traveler also came to the Eliksni and uplifted their civilization.
The Golden Age
When the Traveler came to the Sol System, it used the Light to terraform several planets and moons to render them suitable for human habitation.
When the Darkness pursued the Traveler to the Sol System, a cataclysmic battle ensued in which the Traveler used the light to push back the Darkness, at the cost of rendering itself largely inert. Fragments of the Traveler were thrown far from the site of the battle, with one of the largest landing in Europe. The Light emanating from this shard interacted with the surrounding landscape, creating the European Dead Zone.
While it battled the Darkness on Earth, the Traveler also extended a ray of Light to protect the distant colony ship Yang Liwei from the Darkness. This ray pushed back the Darkness, but could not repel it entirely; the energy of the resulting struggle was so great that a kugelblitz formed, which swallowed the Yang Liwei and its crew.
The City Age
After its sacrificial last stand, the Traveler created the Ghosts, small machines infused with Light, to seek out long-dead individuals and resurrect them as warriors with Light-based abilities. These warriors, first called Risen and later known as Guardians, can manifest weapons and defenses made of Arc, Solar or Void Light, and can return from death with the assistance of their Ghost.
During the events of Destiny 2, the Cabal Red Legion invaded the Solar System and attacked the Last City. Forces under Dominus Ghaul successfully captured the Traveler, and placed a cage around it, thus cutting all Guardians connection to the light. Yet, at least one Guardian managed to reclaim their light from a corrupted Shard of the Traveler in the European Dead Zone, proving that not all light emanated from the Traveler itself. Ghaul attempted to repeatedly gain the Traveler's favor and grant his Red Legion the ability to wield the light. However, the Traveler refused, and Ghaul resorted to outright stealing the light by siphoning it from the Traveler via its cage. Light was only restored to all Guardians after the Traveler broke free of its cage and destroyed once and for all a power-hungry Ghaul.
In Forsaken, it is revealed that the Traveler had been "speaking" to the Guardians through visions of crowns, roses, silver trees, and torches turning into bonfires since its awakening. Ikora Rey tells the Guardians to follow these visions, which leads them to Io. The remnant light on Io then granted the Guardians new powers.
The Nature of the Light
In addition to being a tangible paracausal force, the Light is also synonymous with a certain existential philosophy, as detailed in several Lore entries. The way of the Light emphasizes cooperation between intelligent beings, with the goal of creating "a gentle world" where all entities may harmoniously coexist. Its antithesis is the philosophy of the Darkness, which claims that "existence is the struggle to exist" and that all intelligent life must compete to survive at the expense of other beings. The philosophy of the Light is also referenced in the Marasenna as "the way of the bomb," meaning that it encourages a complex world with many interrelated parts that rely on each other to achieve greater goals. In contrast, the philosophy of the Darkness is described as "the way of the sword", which seeks to reduce everything in the universe to a final, simplified, self-same state.
The Traveler created the Ghosts to seek out those who can wield its light as a weapon. The Ghosts revive fallen warriors as Lightbearers with the intent of protecting Humanity when the Traveler no longer could.
The Speaker views the Light to live in all places and all things. He says you can block it and even try to trap it, but the Light will always find its way. He states that Guardians are called to do what the Traveler cannot and that ordinary citizens now view the Guardians as it’s saviours over the Traveler. Viewing The Traveler to been apart of their past, but present is shaped by The Light and will be their future.
Toland, the Shattered believes that the Traveler came out of the void which is full of power and surrounds everything. He claims that Lightbearers are gifted and armed with the secret physics of a lost age capable of tearing reality apart and concludes that there is one true logical end to the universe which he describes as "The Last True Shape."
The Warmind Rasputin first refers to The Traveler as "the Gardener", alluding to its role in nurturing civilization. However Rasputin grew cautious of the Traveler as it came from somewhere unknown. Due to this Rasputin began to question its motives and whether the Traveler could bring any foreign dangers with it.
Hating the possibility of risks, he devised a contingency plan that would be kept secret and no other human or AI system could access for the sake of protecting humanity. His plan was if the Traveler showed any signs of betrayal or attempts to flee, Rasputin would prevent this as he believed he was morally justified to protect humanity.
Elements of Light
Void is one of several known manifestations of the Light, along with Arc and Solar. It is related to dark energy and gravitational attraction, and can be wielded by Guardians to generate black holes, spatial warps, and other distortions of spacetime. In contrast Arc is related to electricity, and can generate lightning bolts and other electromagnetic phenomena while Solar is related to combustion, heat and nuclear fusion, and generate flames and other heat-related phenomena.
Ikora Rey said that Solar Light is despised by Hive Wizards as it cuts their ritual barriers to shreds, blisters their skin and burns their eyes. Zavala once espoused the beauty of Sunsinger light, until he saw the full extent of its power. Ulan-Tan said to the Speaker that the Light of the Sunsinger is proof that Light is everywhere.
Shard of the Traveler
The Shard of the Traveler is a pure fragment of the Traveler located in the middle of the European Dead Zone. Due the corrupted energy emanating from the Shard, it is the source of the corruption growing around the Blackened Forest. The Shard plays a instrumental role in helping the Guardians regain their Light during the Story Mission Spark. When entering close proximity of the Shard, Ghost claims that he hasn’t been this close to the Traveler’s Light since his inception. The Hive also obtain a shard of the Traveler and conduct a ritual to drain its Light, which was interrupted by the Guardian.
The Fallen civilization once hosted the Traveler, which they named the Great Machine. However, it left their system around the time of a catastrophic event only known as the Whirlwind. The Eliksni also held their Prime Servitors to great esteem and worshipped them as god as they were sources of Life and Light. In a desperate bid to reclaim the Great Machine, they used their Primes to chart interstellar flight and followed the Light throughout the vast cosmos.
Arriving in the Sol System, they began conflicts with Humanity to claim the Traveler for themselves, in an effort to prevent the destruction of their race. While captured Skolas, Kell of Kells tells Variks, the Loyal to have seen a "Light-Snuffer" and "Dark-binder", referring to Oryx, the Taken King. When Skolas dies at the hands of the Guardians, Variks wonders if their own Eliksni heroes were bestowed with Light.
Later a new Fallen House called the House of Light would rise under the Kellship of Mithrax, Kell of Light. He believed that Humanity deserved the Traveler over the Eliksni and proclaim that he will become Kell to the "Mind-Open Fallen" who are willing to fight for "The Great Machine" together with Humanity. On the otherhand Eramis, Kell of Darkness founded the House of Salvation seeking revenge against the Traveler for abandoning her people and to feel their pain through the use of Darkness. She views the Light to only last so long, while Darkness reigns forever.
Dominus Ghaul, emperor of the Cabal, traveled to the Solar System with his elite the Red Legion with the goal of claiming the Traveler and its Light for himself. By attacking the City, Ghaul sought to show the Traveler the error of its ways for choosing Humanity over the Cabal and aimed to establish a new order of Guardians from within the ranks of his own species.
After interrogating The Speaker, Ghaul began to accept that to earn the Traveler's favor he would need to learn devotion and self-sacrifice. The Consul, in contrast, only viewed it as an advanced machine, and advised Ghaul to seize its Light by force. When The Consul refers to the Traveler as "this great machine", it provides insight that the Fallen were indeed referring to the Traveler, as the he notes that other civilizations were precise in their naming. Ghaul disagreed with the Consul's approach which eventually lead to the death of the Speaker and Consul. Having no choice Ghaul took his warship to the center of the Traveler Cage device to drain the Light by force.
Ghaul having successfully stole some of the Traveler's Light managed to become a Lightbearer. However, the Light bestowed upon him was corrupted as stealing the Light is an act of Darkness. Ghaul explores his Light abilities by trying to use Radiance and due to his corrupted Light grows into a massive ethereal figure. He declares himself immortal as he speaks to the Traveler directly which promptly awakens the Traveler from its slumber, destroying the device harvesting its power and Ghaul himself. Its reawakening sends a massive shockwave across the Solar System and restores the Light.
Later the exiled Emperor Calus claims to have knowledge on the true origins and purpose of the Traveler, having encountered powers far greater during his travels. When Val Ca'uor rose to prominence he also agreed with the Consuls sentiments and viewed the Traveler as a weapon belonging to the Cabal.
Due to the paracausal nature of the Light, the Vex are unable to simulate it. While they may succeed in projecting a image of the Traveler, they are unable to actualise it's power and fear this nature of the Light as it is beyond their understanding. While they can determine the destructive outcome of a Nova Bomb, they are unable to simulate if the Guardian will use their powers at all.
In the the Vault of Glass, a pocket universe they controlled and could write things out of existence, they were able to create ontological weapons such the Gorgons and Oracles. However they could not foresee the creation of the Aegis by Kabr, the Legionless, who pierced a wound in the Vault to let the Light in.
When Saint-14 entered the Vex network, they spent years tuning a Mind to the frequencies his Light. Eventually Agioktis, Martyr Mindis would hunt down the Titan out of desperation to stop his rampage. While Saint successfully destroyed the Mind, it succeeded in draining his Light and leaving him helpless for the Vex to finish off. Out of respect, the Vex laid Saint-14's body to rest in a tomb as a memorial to the grief he gave them. The great respect the Vex have for Saint-14 is symbolised in the naming of Hagios and Agioktis as both mean Saint in Greek. Though they are not worshipers of the Light, they showed admiration for its mightiest of Lightbearers.
Awoken and Jovians
The Ammonites worshipped a song developed by the Traveler, dubbed the “Song of Life”. The Hive attempted to sing the song in reverse, which in turn killed the Choir and lead to the creation of the Deathsong.
During Year One of Destiny, Light was a value present on all level 20 armor of Rare, Legendary, and Exotic rarity. In order to increase a character's level beyond 20, the player had to obtain armor with more and more Light, which would give them a Light level between 21 and 34 depending on the Light value of the gear. The highest attainable Light level in Year One (after the release of the House of Wolves expansion) was level 34, which required four armor pieces (one for each armor slot available at the time) with 36 Light each, which came to a total of 144 Light.
Light level determined the player's strength and what activities the player could participate in. To participate in an activity effectively, players had to have a Light level that matched or exceeded the activity's recommended Light level, or the higher-level enemy units would be able to both inflict significantly more damage on the player and resist more of the player's attacks. If the player was as little as four levels below an enemy, the player's damage output would be reduced to the point that the enemy would be "immune" to the player's attacks; the player would actually still be damaging the enemy, but the amount of damage being inflicted would be so small as to render killing the enemy within a reasonable amount of time an extremely difficult, if not impossible, feat.
The Taken King changed the way Light was determined and used by players. Light is now a weighted average of the Attack and Defense values of all of the player's currently equipped gear; weapons count more heavily in the calculation than armor. Under this new system, Light reflects the player's strength and what activities the player is prepared for, but operates on a less severe penalty curve than Year One Light levels; whereas a Light deficit of four would make the player harmless to an enemy in Year One, in Year Two it takes a much higher Light deficit to noticeably impair the player's combat effectiveness. The Attack and Defense values of all Year One gear is capped at 170, meaning a player must equip Year Two gear in order to reach a higher level of Light.
Certain thresholds of Light must be attained in order to unlock or be able to complete some activities: Light 240 unlocks the Daily Heroic Story, Light 260 unlocks the Vanguard Heroic Strikes playlist, and Light 280 unlocks the Nightfall. The Raid King's Fall is available immediately, but it is recommended to achieve a Light level of 290 before attempting. The highest attainable Light level in Year Two was 335.
The Light mechanic remained the same as Year Two with the transition to Rise of Iron, but the limit was raised. Light 300 unlocks the Daily Heroic Story, and Light 320 unlocks the SIVA Crisis Heroic Strikes playlist and the Nightfall. The Raid Wrath of the Machine will have a recommended Light level of 360 on normal difficulty, and 385 on heroic difficulty. The highest attainable Light level in Year Three was 425.
The Light mechanic from Year Two of Destiny was carried over to Destiny 2, but was renamed Power. Everyone starts at 750 Power, and can raise their natural Power up to a maximum of 970 with the introduction of the Season of Dawn, then augment it further by leveling up the seasonal artifact. However, soft caps make reaching 970 power with gear difficult. While all engrams will raise a player's average Power level until they reach Power level 900, and Prime Engrams, Exotic Engrams, and powerful rewards will take them to 960, only pinnacle rewards will be able to level a player from 960 to 970. There are only a few sources of pinnacle rewards, each of which can only be acquired once per week, and those are:
Most content in Destiny 2 currently has a recommended Power level of 750, effectively meaning they have no Power requirement. The exceptions are content coming with and following Shadowkeep, such as its campaign, Nightmare Hunts, the Pit of Heresy, the Garden of Salvation, and the Sundial. Additionally, Nightfalls can impose a Power handicap through the Five of Swords (purchasable from Xur), and Ordeals, Nightmare Hunts, and the Sundial all have multiple difficulties, ranging from 750 Power to 980 Power (among other differences).
List of appearances