Mimics (The Formless)
Mimics; often called the formless because no-one living has seen their true face. Rumours say that the mimic will show that to its victim, moments before it kills and replaces them.
That is what they do best, replace people.
Ever noticed somebody suddenly become slightly different? They look the same, they act the same (mostly), they know you just like always but sometimes...they seem to have forgotten things, nothing vital or terribly important but miggling little details... It is possible that your friend is dead and a mimic is watching you. Beware because the dead whom they replace are never the mimic's true target, that was simply done to allow it to get close enough.
Like striga-strain nephilim, mimics are pathovores (they feed on emotions). Mimics will suck the victim dry of everything which makes them - well - them. Memories, secrets, everything. Are you familiar with the popular saying about one's life "flashing before one's eyes" at the point of death? The mimic waits for that and drinks it down like a fine wine. The mimic will then sleep in order to affect the physical changes as well as integrate memories and personality into their minds, often in presence of the recently-deceased. Once ready they put on the corpse's clothes and install themselves in their lives.
Mimics are completely undetectable by today's technology. Some sensitives and animals can detect them or at least feel terribly uncomfortable in the presence of one; something indefinable which just feels wrong. Beware because if the mimic's mission is important enough, they will find ways and reasons to eliminate anyone who even looks at them with too much curiosity.