spoilers:
here is the wiki story plot for the ending
After defeating the Crimson Lance defenses and alien Guardians, the player opens the Vault with the Vault Key. Once inside, the player must defeat a creature known as the Destroyer. The ancient Eridian aliens had sacrificed their entire civilization to imprison the Destroyer in the Vault, which left unchecked, would have consumed the universe[citation needed]. The Guardians were created to guard the Vault and ensure no one opened it. Once the Destroyer is dead, it is revealed that the Guardian Angel was actually an entity that resided in a satellite orbiting Pandora, and is related to the Claptrap robots. On Pandora, one Claptrap's camera-eye turns orange, and the words "Claptrap: Interplanetary Ninja Assassin" are shown. Afterwards, the player can sell the Vault Key to Patricia Tannis; the Vault has resealed itself and the Key cannot be used again for 200 years, but Tannis still values it. At this point the player can finish any remaining quests or begin a second playthrough, where all enemies will be reset and strengthened to match the player's level.