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 was created by Ward specifically for the story.
I'm looking for how to work out appropriate 5E statistics for the monster in question (the author calls it a Coraltan) based on its actions, abilities and description as depicted in the story.
This '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), presumably through some form of illusion magic such as disguise self.
In the act of preforming what appear to be temple services, this being preformed magic that healed two people badly wounded and weakened by the attack of a vampire, though no specific divine spells as you would find in the Player's Handbook were mentioned by name.
Near the end of the story it also used a form of mind-influencing magic to ensure any humans in its 'flock' would not enter its temple quarters and possibly discover its true nature.
Finally, the monster in question seems to 'feed' on the energy (yes, I know you're going to complain how vague that sounds but it's as specific as I know how to say it) of its human followers in the act of preforming temple services. Rather than drain an single human dry, it seems to absorb a little bit of energy each from individuals in a large crowd, also it appears capable of feeding on a living being's energy without having to reach out and physically touch its victim (again, as specific as I know how to say it).
I would suggest a CR of at least 10, possibly closer to 15.
What source material (published monsters, class features...) would provide most of the features I cited from the story? (undead, dessicated / worm-infested, take shape of living humanoid, healing and mind control magic, drains life energy) Ideally an example monster would provide most of these, have the appropriate CR already, and allow for the addition of the others without much mechanical rebalancing.