The Gore Phoenix
These strange birds of the underworld have a robust, yet graceful form as they stand five to six feet tall. They have a large and flowing plumage covered in "eyes" in shades of white, black and grey, and usually incorporating a single color. Their feet are covered in terrible claws, and their long curving beaks are as sharp as the greatest of blades. While they do move in packs in the Underworld and are known to attack ghosts and the living alike, this is not the greatest terror that they bring.
Though savants and scholars alike are unsure of the method, it seems that in recent times, the Gore Phoenix has lived up to it's name. It is able to somehow "infest" a human host, slowly growing as a mostly immaterial being, extending throughout the body slowly, until the host no longer has control of the body. The bird now has full possession, but this is only seemingly a part of it's life cycle. It sometimes acts as a man insane and filled with bloodlust. It is unsure, but it is believed that this is part of their cycle of reproduction. Then they can "infect" other humans with a spirtual egg that will eventually hatch and take over it's host also, but the method of this is unknown as of yet as incursions are still relatively rare. Eventually, the person is killed, and the body disintegrates into gore from which the bird bursts, fully grown and materialized.
They can be "killed" as any normal being of flesh and blood, but they become as a spirit and reappear, hurt but immaterial. It is not known why these birds have suddenly developed this ability, or why they do it, but there are some clues scholars point to in evidence. The name of gore phoenix is one that seems to have existed for a number of years, perhaps even in the time of the Old Realm. This implies they have always had the ability, but have been not using it till recent times again. There are theories that they are one of the elementals of the Underworld, most likely of blood.
They can be removed through mystical methods before they have fully taken over a person, but attempting such afterwards results in a slow, horrific death and the birth of the material beast as normal. It is believed that they have subsumed quite a bit of the blood and similar organs of the body, and that when forced out, these things are taken with it. There are magical ways of quickly saving the body... but then mind and soul are trapped in a body that they can never control (without a magic capable of healing the damage to the connection between the soul and the body).