A lifeform that arrived in the core of a meteorite. Unlike the distantly related Genetisaur, all Underfiend are genetically male, and target females as hosts to their parasitic genes. Unfortunately, its attempts to replicate itself will usually result in little more than a worthless, barely-living deformity, and even healthy offspring are frequently born sterile. Throughout gestation, hosts suffer an unbearable craving for the blood of their own species, and even develop a leech-like tongue similar to certain vampires.
Over time, Underfiend can adapt to local customs and demonstrate more civilized, more considerate mating behavior. One particular specimen even enjoys a successful career in marriage counseling.
Yes, this is exactly what you're either afraid or hoping it is, depending on the person. I tried to put it gently. Mortasheen stuff will never be much more explicit than this description, and always a little lighthearted no matter how horrible the idea really is underneath.
It's only fair, considering how many monsters I have that are siren-style man-eaters.