In Cult of the Lamb, you can turn followers into demons to help you during a crusade.
What happens to the follower? Do they return to being a follower after? Die? Something else?
Once converted, Followers don't work and persist as a demon until the player starts a crusade run. They cannot be killed and can provide very valuable support if used properly. Upgrading the Circle allows additional demons to be summoned for a run, with a maximum of three.
When returning from a crusade, demon followers return to their normal form and will be exhausted.
Source, Demons.