“Contract AU” features characters who appear as demons, angels, or reapers, based on the nature of their souls. Humans can also be turned into any of these beings, depending on how tainted or purified their souls become.

The setting takes place in an alternate version of modern earth, where these supernatural beings disguise themselves and live among humans and each other. The verse’s namesake comes from a demon’s ability to form contracts with humans — often in exchange for their soul, but sometimes as a part of a deeper bond.