This is the ‘Big Bad’ of the short story Undefiled, written by James M. Ward for the collection Tales of Ravenloft. From what I’ve inquired the creature in question was created by Ward specifically for the story. The monster in question (the author calls it a Coraltan) is a corporeal undead whose body is described as “desiccated and worm-infested”, however it is able to take on the appearance of a living humanoid (in this story it poses as a city’s spiritual leader) and while implied to feed on the energy of the living- in the story it seems to draw a little bit of energy from each person in a large crowd-, also appears to be capable of healing magic as well as mind-influencing magic (which it uses to keep people from its proverbial flock from discovering its true nature).
I’m not sure of the best way to simulate these qualities in 5E mechanics. At first I considered giving it the ability to cast Disguise Self at will (to appear as a living being rather than undead) except that spell has duration of an hour and is described as a wizard, sorcerer or bard spell and the monster’s healing abilities suggest divine magic- in 5E would an undead being like this be even capable of handling positive energy? Also what would be the best way to describe how it can feed on people without necessarily physically touching them?