Possession

A 'shadow' . Can turn into dead things they've ate. If one consumes something alive they take that form and are unable to turn into anything else.

Original body owner was on verge of death and made pact with the fmc. For her to eat her alive and take her identity she wanted the shadow to protect the 2nd prince

Demon. The original fmc's body was poisoned and wanted revenge-made contract with demon who then took over her body