Deep scion

Deep scions were people who surrendered their bodies and souls to an evil aquatic power (usually a kraken) in exchange for their lives. Upon surrendering, they were transformed by a ritual into amphibious shapechangers, in service to their masters.

Description
Deep scions believed that their true form was the one they assumed when they returned to the sea. However, when killed, a deep scion's corpse would revert to the form it was before its transformation.

Society
A newly transformed deep scion would usually return to its former home while appearing to be the person it used to be, but might also take a new identity. However the manner of its return to the surface, the deep scion's only purpose was to infiltrate into humanoid society in order to carry out its master's wishes.