So two weeks ago I started talking about a monster type I made called the Edere Vir. Originally envisioned as a part of the World of Darkness, they were humans that preyed on other humans for power and advantage within society. I tried to adapt them to a D&D mileu without success.
Initially. In D&D, the Edere Vir would be the town baker who finds a wounded traveler with a broken leg and is overcome with a taboo desire. They give in and find their old bum knee is healed. Much to their own horror, they feel neither revulsion nor remorse.
Time passes and their small village sees travelers savagely attacked. The local mayor--a former baker--reluctantly calls the party in. The adventurers eventually discover the baker-cum-mayor has been quietly trying to cover the entire business up. That the limping baker is suddenly more than fully ambulatory is a surprise. That they can bite through the fingers of their fellow elf as though they were made of butter is downright shocking to the party.
The party triumphs and the baker is publicly hung by the neck until dead and interred with all appropriate rituals. The party calls it a side-quest and rides off into the sunset.
Months pass and the adventures is forgotten until they meet an exchequer in the service to a hiring noble. He wears an ascot or a scarf at all times--an old fashion, but one still observed amongst a number of those who wear such fineries. The exchequer seems unfamiliar with them (lying powers) and a few Sense Motive checks later it seems like a harmless resemblance. Of course, they still have the noose impression on their neck (Regeneration is regeneration, but death leaves a mark) and they still bear a grudge. Credible (again, more lying powers) enough to turn everyone against the party, they are now a far cry from the small-minded baker the party once foiled. Not merely a practiced and experienced Edere Vir, they're a regarded servant with the ear of a noble and ambitions to something greater.
At this point, the rash of murder victims with their necks eaten out begins to make sense; the baker has been trying to heal the one thing they can't. The party engages the Edere Vir and learns that they can regenerate body parts they've eaten, fake royal orders (copying handwriting by eating hands), and perform extraordinary intellectual and physical feats (basic stat boosting by eating muscles, brains, etc).
The party understands how the baker can use what they eat so when they corner their adversary to find nothing but a peasant body with a missing face, it's all the more ominous.
Now the threat is everywhere. Any new nobility or rich merchants are suspects. Potential hirers or new recruits. Even old acquaintances could have their faces eaten off so that the Edere Vir could get their vengeance. The noosemarks are the only real clue, but how hard is it to conceal them with make-up or magic
 An ability to bite through the bones, sinew, and flesh of their own species is, by necessity, one of their powers.