A long time, depending on who you ask, They sought to undo all of creation. Rising up from The Far Realms, They began to pour across the boundaries into reality and started unmaking. Reality, no longer in its infancy, rose up to defend itself as the very spirits of what makes the planes took mortal form to enter the fray.
The war raged on, and no one was sure who would rise victorious. It was during this time that the Vessels rose to power. Great heroes of their age, the Vessels sacrificed themselves and their civilizations to cast Them out and erect a mighty barrier to protect reality.
Nothing lasts forever, though. And The Far Realms are beyond time. They are returning; and this time, They are more prepared than the last. When the forces of reality chose mortal forms, They intervened. Cast adrift through the planes, these new, Corrupted Vessels have no idea who they are, what came before, or what is coming next.