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?
Best Answer
How you unmask any cultist
As spells go, detect thoughts (a 2nd level spell) will help, just as half of other divination spells might too. I am going to focus on even more basic techniques, though.
Cultists, generally, look like normal people (because they are), just like Deep Scions do (when polymorphed). What makes them different is their behavior. They meet up, scheme, plan, leave dead bodies and strange diagrams in their wake. They visit forbidden places, act out forbidden rituals, keeps secrets.
A smart PC can detect this through careful observation, snooping around and meddling and being intusive. Maybe even provoking their hostility. Once they have enough evidence, they go for the kill – either literally or figuratively.