From Destinypedia, the Destiny wiki
The Infinite Forest is a location accessible through a gateway in the Fields of Glass, Mercury. It is a planet-sized machine that simulates trillions to uncountably infinite number of alternating realities which the Vex plan their actions out in the current timeline. The Vex hollowed out Mercury's core, and in its place built the mechanism that generates the Forest. It was overseen by Panoptes, Infinite Mind until its destruction. Simulated versions of Dendron, Root Mind created by Osiris have since attempted to take control of the Forest, but have been destroyed by The Guardian before they could assume the role.
The main entrance of the Infinite Forest can be entered via a triangular portal in the Fields of Glass. This portal leads to a "debug chamber", which manifests as a short hallway with portals on either end. While passing through this chamber, a visitor may notice waves of light scanning across their body, where they pass from regular reality to the Vex Realm.
Traversing the debug chamber leads to the "hub" area of the Infinite Forest, located in what appears to be either the actual interior of Mercury, or a simulated version of it. This area contains four tower-like structures with gateways, three of which lead to different time periods within different simulations, and one that leads back to Mercury's surface. The sleek, futuristic tower to the left of the entrance leads to the distant past where Mercury is a lush garden world; the tower directly across the entrance, which is composed of conventional Vex architecture, leads to iterations of the present; and the crumbling, darkened tower to the right of the entrance leads to the far future, where the Sun is long dead, along with all life and the Vex have spread everywhere. The Infinite Forest does not only simulate Mercury or the Sun, but also entire timelines, comparable to other simulations of universes, there can even be doorways to other places such as Mars, or even travel from Mercury to Earth. As Guardians move toward the main towers, they encounter simulated Cabal, Fallen, and Hive, as well as Vex, and must defeat disguised units called "Daemons" to generate new pathways and advance, these pathways change randomly every time one enters the forest.
Within simulations of the past, present, and future, Guardians will continue to encounter simulated enemies from the other races. For example, in one of the infinite variations of reality being analyzed, the far future where the Vex have triumphed over everything may also contain Fallen survivors. The Forest can even simulate realities where the Infinite Forest did not even exist.
The Vex can use the Infinite Forest to understand enemy strategies and tactics, such as the Fallen's patterns of retreat. The Vex can manipulate virtually any variable within these simulations, including how long the simulation itself lasts or whether it repeats itself. They can even set such simulations on timers for however long they last. and within such space, they can alter the Geometry. Osiris comments on how fascinating the simulation is with it's combination of constants and variables, rigid structure in movement, dynamism in engagement, everything within a strict framework.
They can focus a lot of their processing power on particular simulations in the forest, whether it is to test for a perpetual feedback loop to establish a benchmark dataset or it certain individuals (Cabal individual) is vital to their strategy. Even when running tests based on enemy activities, they can still retrieve the data from the tests to determine their next move.
Though the Forest is infinite in so many ways , they also pertain to victims such as the Red Legion trapped there in loop as well, seemingly for infinite eternities.  Also the geometric data of the Forest’s architecture subtle shifts with reality trying to decide what version it wants, and time is constantly in flux as well. And depending on who has an understanding of Vex tech, you can enter or leave the Infinite Forest without any physical gateways. Osiris also said that the Infinite Forest must not be destroyed no matter how much we want to as it would cause detrimental, damaging side affects in our timeline.
The Simulation Blooms and Simulation Seeds are physical matter that can be found in and outside of the Forest, with the latter being what makes up the Forest itself. Even the Blooms that were part of the Forest can split off into their own self-contained realities. Even when simulating the Hive, it still managed to replicate the same terror that a swarm is capable, and that Osiris admitted the Forest made him realize it's threat. Osiris and Sagira even attempted to simulate the Hive's Homeworld, despite how the Guardian's Ghost compared their homeworld to the Forest based on the number of Hive. The Ghost even mentioned how Oryx wouldn't need an Ascendant realm if he had the Forest, were as Osiris debunked saying that Oryx would always need a Ascendant Realm in a Simulation when he created it without it. And when there are signatures found in the Forest that are the Hive, they are the same as the ones found in the New Pacific Arcology on Titan.
The Golden Age
The construction of the Infinite Forest engine began during the Golden Age, while Mercury was still a garden world and the Vex had only recently arrived. The algorithm used to generate the Forest was housed within the Gardener's Spire, overseen by Dendron, Root Mind. Panoptes, Infinite Mind was tasked with managing and interpreting the Infinite Forest's simulations to inform the Vex's strategies in the "real" universe.
Following his defeat of Solkis, Devil Kell, Saint-14 received word that Osiris had ventured to Mercury to investigate the Vex's activities. Fearing that the exiled Warlock would instigate a full-blown war with the Vex, Saint-14 followed Osiris to Mercury and went looking for him in the Infinite Forest. For years He battled a horde of Vex within the Forest, and was eventually confronted by Agioktis, Martyr Mind, a Vex Mind specifically designed to drain his Light. Though Saint vanquished the Mind, he succumbed to his wounds and died his final death; the Vex constructed a tomb for him within the simulation, seemingly as a gesture of respect.
Curse of Osiris
After the Traveler released a pulse of Light at the conclusion of the Red War, the gateway to the Infinite Forest reactivated. Osiris and Sagira while exploring the forest in some part of it’s infinite realities where searching for ways to defeat the Vex. During a simulation of the Vault of Glass, Osiris peered into the possible futures that the Vex had conspired; what he saw startled him that the Traveler's awakening gave way to an imminent outcome for the Vex to succeed in their goal of Convergence. With Panoptes' gaze able to see Sagira’s Light, Osiris left no choice but to sent her out of the Forest. Despite suffering injuries, she was successfully recovered by Ikora’s Hidden. The Guardian would then take Sagira’s Shell and they would travel to the Infinite Forest to figure out what is going on. They realized that they could not get in unless it was Sagira’s ghost, her Light would later fuse with their ghost's and gain access to the forest.
When inside, the Guardian encountered Light-constructed reflections of Osiris, they fought their way through Mercury’s past, seeing how the Vex corrupted the planet, and to the dark future that Osiris saw. They then realized that they had to defeat the Axis Mind Panoptes in order to forestall the future from happening.
They went to Io’s Pyramidion to try and get a map to Panoptes’ location in the forest, they then went into Mercury’s past to get it’s location then before the forest was made. Panoptes would appear and capture Sagira and expel the Guardian from the forest, Ikora would then assist the Guardian in entering the forest again, where they would finally confront Panoptes one more time, with the aid of Osiris, preventing the Guardian from being erased, they managed to defeat the Axis Mind, with Osiris and Sagira returning to the forest and under new management,
Later on, the Guardian would return to the forest to aid Osiris and Sagira with two issues that have sprang up. With Osiris recreating the Dendron, Root Mind that gained sentience and attempted to regain control of the forest. And a Cabal invasion glitch where they attempt to steal world ending tech, the Guardian promptly ended both issues.
While the City celebrated the Festival of the Lost in the year after the Red War, the Infinite Forest began exhibiting strange behavior, taking on a new iteration of the Forest where simulations prey on the fears of others, with the primary simulation area within Mercury's core becoming darkened and inhabited by simulated entities known as "Nightmares". Later, A part of the Forest started to exhibit an environment of lush flora, referred to as the Verdant Forest, greenery during the Revelry.
Shadowkeep/Season of the Undying
Season of the Dawn
Osiris would later return into the Forest to monitor it's simulations, and as he and Sagira entered the realm, it appeared pitch black. Thinking something may have gone wrong, they contacted Ikora. She informed them of the war on the Moon and the Vex invasions, and that the Guardian ended The Undying Mind. Osiris in turn, notified Ikora that when the Mind was destroyed it changed the course of history and futures.
As the duo returned back to their current position in the Simulation, on Mercury, everything was pitch black, there were no stars, and the void reached to the Traveler and likely beyond. The purple twilight of the Sun from the Dark Future was unseen and the shimmer from the Forest was gone. Arriving at The Last City, they found that there was no Traveler, in it's place was a Large Obsidian Monolith at least twice the size of the City. Osiris projected that this would likely come to pass in about two to three decades depending on a multitude of variables, with acceleration potentially caused by the actions of the Guardian.
With the incursion of the Red Legion lead by the Three Psion Sisters, they sought a reality where they won the Red War, one of them included either the usage of the Infinite Forest or it's destruction, either case, Osiris wanted the Guardians to stop them or it would have detrimental affects on the timeline.
We would later use the Sundial to save Saint-14 from the Infinite Forest, changing the past and preventing his death, he would later exit the forest at some point in Mercury's past and return to the City to defend it and learn from the history he was absent from.
Later, the Guardian would return to Osiris and he would thank them for cleaning up his messes and congratulate them on their rescue of Saint-14. He would explain what he had witnessed in that future, he had saw the destruction of the Universe at a sub-atomic level, nothingness remained, he had hoped that our actions with The Sundial would have prevented this, for he believed that no power or gun could stop that emptiness.
Season of Arrivals
With the Festival of the Lost, Eva Levante sends the Guardian to go see The Spider about helping him with some favors. He mentions that at some point, the House of Kings led a salvage operation on the Dreadnaught and while on the way back, the Vex slew the salvage crew, managed to simulate their salvaging party, and were able to take what the Fallen had found. The Guardians were tasked in heading to the Forest to retrieve what the Vex had been simulating.
At the end of the Season of Arrivals, after Zavala issued evacuation orders for all contacts on Pyramid-sieged planets, the Guardian set off from Mercury, leaving Brother Vance alone. He made his way from the Lighthouse to the gate of the Infinite Forest, entering with his Infinite Simulacrum, attuning it to the crack in the Forest itself, and sealing it from within.
List of appearances