The Destroyer intergrates the intelligence of those it devours, and understands the knowledge and cultures of its victims, making it all the more difficult to capture. The Eridians sacrificed themselves to trap the Destroyer inside the Great Vault.
Typhon de Leon defeated the Vault Monster on Nekrotafayo.
On Nekrotafayo, there's a machine designed to close the Great Vault, called the Machine.
The Machine uses astrokinetic energy to draw power from the Vaults in order to close the Great Vault.
The Machine is the size of a planet.
The Vaults on other planets power the Machine.
As all the Vaults and the Vault Keys power the Machine which seals Pandora, and the Vault Monsters are the power of the Vaults, this means that each Vault Key would be planet level. So not only would this scale to the Rampager and the Graveward, but also back to the Warrior and Borderlands 2 Vault Hunters, and Handsome Jack.
The Machine is what sealed the Great Vault the first time around.
The Machine has enough power to seal Pandora.
Tannis channels the power of the Machine.
After this, Tannis says that she has mastery over time and space.
Tyreen appears and Typhon tries to take her down with a bomb, but she catches the explosion and deflects it back at him, killing Typhon.
As the Machine works across interstellar distances, Tannis is able to manipulate it to create a portal to Pandora.
The Vault Hunters go through the portal back to Pandora, where Tyreen absorbs part of the Destroyers power and becomes a monster. They defeat her and Lilith regains her powers, but Elpis is still falling, so Lilith sacrifices herself to save Pandora and prevent the Destroyer from truly escaping. Liliths sacrifice puts Elpis back in its place and lights up the entire galaxy (and actually shakes it, if you look at Promethia). Once the light is gone, there's the brand of the Firehawk left scarred on Elpis.
The Eridian artifact in the Vault of the Destroyer allows the Vault Hunters to open up new chaotic shards of reality.
Tannis comments that the Guardian AIs trained themselves either in what are either simulations of more chaotic versions of reality, or outright parallel universes.