When Bungie first launched Destiny in 2015, players were presented with a fairly simple good vs. evil story. The players take on the role of Guardians, once dead people resurrected by a benevolent alien machine called the Traveler. Using the Traveler's light to wield incredible powers, the Guardians are immortal protectors of humanity, watching over the Last City on Earth following an apocalyptic event called the Collapse. The Traveler's enemies are various alien races who either worship or are aligned with a force called the Darkness. For years, the Guardians presented a unified front against the Darkness, but cracks have been forming which may lead to an all-out civil war.
The blemishes in the Guardians' history began to appear subtly with the reveal of certain bits of background lore. For context, the Traveler is a small, white moon that appeared in the Earth's solar system in the 21st century and made first contact with humans on Mars. It proceeded to share its knowledge and advanced technology, leading humanity to enter a Golden Age that cured poverty and inequality, lived for centuries and expanded across the solar system.
Humanity was just about ready to venture out into the stars when the Darkness arrived. It nearly destroyed everyone, but the Traveler saved them, wounding itself in the process and coming to rest over Earth. It used what Light it had to create the Guardians, who eventually founded the Last City beneath the Traveler. Except, that's not quite what happened.
During the Golden Age, humanity built a massive defense network supercomputer, the warmind Rasputin. Its sole purpose was to defend humanity from external threats. Guardians uncovered lore revealing that Rasputin had predicted that the Traveler was preparing to flee the Darkness when it arrived to lay waste to the solar system.
Rasputin was the first to detect the arrival of the Darkness and knew the only thing capable of keeping humanity safe was the Traveler, so it used a secret subroutine to threaten the Traveler -- if it left humanity behind, Rasputin would fire every weapon at its disposal at the Traveler, assuring no-win situation. If Rasputin's reports are to be believed, then the Traveler only stood by humanity because it had to, and the Guardians seem more like a form of self-protection and less like an altruistic gift.
The Guardians also learned there is precedent for the Traveler abandoning those it visits and gifts with knowledge. One of the enemy factions, the Fallen, was once known as the Eliksni. They arrived in the solar system in pursuit of the Traveler, whom they worshiped as a god. However, the Fallen were abandoned by the Traveler and have colonized the solar system, jealous of the humans who have stolen the Traveler's love.
Things got even murkier with the introduction of a character called the Drifter. An outcast Guardian, he returns to the Last City and presents players with a new plan: fight Darkness with Darkness. Through the game mode Gambit, players help the Drifter gather Darkness that he is storing for reasons yet to be revealed. He is tolerated by Vanguard leaders Zavalla and Ikora, but they are wary of him and his true intentions.
In fact, the Drifter presented players with the first (and so far only) moment in the Destiny franchise where they had to make a narrative decision. Players either had to agree to spy on the Drifter and report back to the Vanguard or help the Drifter cover his tracks and keep the Vanguard off his tail. Bungie has promised this choice will have ramifications but hasn't quite delivered on that yet. Right now, if you choose to spy on him, the Drifter gets wise to it and starts calling the player "snitch" during Gambit.
The Shadowkeep expansion furthered the rift by having a Pyramid, a vessel of the Darkness appear for the first time. It also re-introduced Eris Morn, a popular character who has significant ties to the Darkness. Once a Guardian, her Ghost was killed and she lost her connection to the Light. Her whole fireteam was killed in a raid to fight a Hive prince, Crota. Trapped for centuries and badly injured, she was forced to replace her eyes with one of the Hives'.
While loyal to the Guardians, they are likewise wary of her for her fascination and obsession with the Darkness. This has been a focal point for the recent Season of Arrivals, where the Darkness is beginning to show up in force and Eris is attempting to decode their messages. Zavalla has shown concern over Eris' unrelenting focus, and players have learned that she is now in league with the Drifter, believing that drastic measures will be needed to fight the Darkness.
Season of Arrivals also brought a new exotic weapon, Ruinous Effigy, a gift to the players from the Darkness. In Shadowkeep the Darkness spoke to the players for the first time, telling them that they brought enlightenment, not destruction. Eris has dialogue in the seasonal "Interference" missions that posits the Traveler is a boundary humanity must break through to thrive. The Darkness seems to be trying to seduce the player, and it might work.
This is all a prelude to the November release of Destiny 2's Beyond Light expansion, which among many other changes, will be adding a new powerset to the game-- the Darkness itself. Players will gain access to the new, ice-themed Stasis powers, which are a manifestation of the Darkness. It also promises to address a long-standing mystery in the Destiny universe-- the Exo Stranger.
She is notorious for stalking the player, appearing in the background at pivotal moments, and, when she does approach players, she only provides cryptic nonsense. We know she's not a Guardian, but a time traveler from a future where humanity fails. She believes the players are the key to changing things. Not only is she back for Beyond Light, but footage released so far leaves the impression that she too is aligned with the Drifter and Eris.
The three of them all have relationships with players' Guardian, and Eris is even considers the player a friend. Tensions are mounting, and Bungie has promised that the player's choice of siding with or against the Drifter will have ramifications. The Darkness is the antithesis of the Guardians' Light, wielding it as a weapon, even for a good cause, might very well lead to a rift that could devolve into civil war.