Black Bones:

Black Bones are a type of Skeleton monsters that are an amalgam of several different vengeful spirits, forcefully collected into a Skeleton.

These creatures are a walking mass of grudges, and they will kill anything that they lay their eyes on, Just touching a Black Bone can infect creatures with the grudge of the vengeful spirits, making them a threat to any living being on the planet.

Only Skeletons are the only type of undeads susceptible to this type of possession, as the rotten flesh of zombies becomes too unstable as a vessel for the vengeful spirits, and ghosts are already a type of spirits. 

Traits and strengths:

The strength of the Black Bone is determined by the Skeleton monster the vengeful spirits possess. The weaker the Skeleton, the weaker the Black Bone. Still, due to the possession of the vengeful spirits, the power of the Black Bone is many-multiplied with the original value. On the other hand, any intelligence the Skeleton may or may not have had is gone, replaced with an everlasting bloodlust.

The grudge that contaminates the Black Bone is contagious. Just touching the weakest Black Bone is enough to kill a creature with less-than-average resistance towards curses. Still, they are weak against long-range attacks, as whatever skills or abilities the Skeleton may have had before turning into a Black Bone is lost due to the possession, making them a perfect target for magicians and marksmen. 

The worst aspect of a Black Bone is that it releases the pent-up grudges in an exploding manner when the Black Bone is defeated. If one is not careful when dealing with Black Bones, an area could be contaminated by vengeful ghosts for decades.

Garami's comment: See! It's that bad!

Mira's comment: Stop talking and start stoping! If that thing blows up, this won't end with the end of our free lives, but also our wallets!!