Based on the current information we have:

Twenty-four hours after the Demon Emperor obtained the Resurrection God Stone from the Gate of Immortality in the Forty-Fourth World, ten new types of monsters, previously unrecorded in history, began appearing across various worlds in the Otherworld. These correspond to the number of Resurrection God Stones the Demon Emperor acquired.

It is certain that these creatures were resurrected by the Demon Emperor using the Resurrection God Stones, and they have a unique trait: they must attach themselves to Professionals to survive in the Otherworld, essentially possessing them!

Moreover, their strength surpasses that of King-level BOSSes!