DD/: TheENDtale (Undertale AU Fanfic)

Error won.
But his victory wasn't what he expected, fused with the lone survivor they fought off the demon that tricked him. But now alone in THE END, they work to rebuild everything they lost, at least until they are offered redemption by a strange entity. They cannot change their fate but if they help an Error from another reality, they can stop the past from repeating itself.

But that is harder than it sounds, especially since the demon has other plans.