Destiny is an online "Shared Shooter" set in a scifi post-apocalyptic future based around a single city where most of the human population lives because of an event called the "Collapse". No one knows what caused the collapse, but there are alien forces constantly trying to kill each other and the player character.
The player character is part of The Guardians that protect the city, and are granted powers by the space entity called The Traveler. The Traveler is seemingly dead during the events of the game, but previously it sacrificed itself to save humanity from it's nemesis just dubbed The Darkness.

[: The lore is pretty awesome for the game, which is what I love the most about it.