The deep scion is a monster detailed in the fifth edition supplement Volo's Guide to Monsters (p. 135). I'm unaware if deep scions have featured in previous editions of DnD.
Deep scions are people who have been subjected to a ritual that transforms them into an evil shapechanger capable of:
wearing the mind and body of the person it once was as a sort of mask.
Imagining that a coastal village has been infiltrated by deep scions, who are stealing away residents and assimilating them one by one, once the wider threat has been discovered (perhaps a group of deep scions surprised while in piscine form) how would other individual deep scions be unmasked?
Deep scions seem to be extremely capable of maintaining their humaniod disguises in the long term, so having dealt with / questioned all of the obvious candidates how would PCs know whether the threat had been neutralised?
Is there any relevant deep scions lore in other supplements, or even other editions, that might provide some transferable guidance?