That devouring chest (Patreon)

In area 12 on level 1 of the Tomb of Annihilation is a chest. It has a tiny, perturbing inscription hidden around an inlaid silvered demon skull that reads: "I devour all but the greatest thieves." Inside the demon's maw, behind a dozen needle sharp fangs, is the keyhole. 

This is a player handout to convey the seriousness of the lock. ;)