Wyrths are very rare, but equally dangerous. They are created in foul rites (or elven parties, or in the revelries of the faerie, or in orcish stillbirths, or from cursed maidens; there are a lot of tales around) that involve cutting a living creature to two pieces, while it is still alive.
A wyrth can be made of numerous creatures, but wolves are the standard. After the ritual has been completed the both pairs of the wolf are given a sniff of blood of some unwary victim. One or two targets. The wolves will hunt their prey down tirelessly and kill it without hesitation.
And they are quite good at it. They can walk unseen, and usually do. They close to the target and jump for the throat, usually immediaely. Some have stalked their prey for some time before killing it. And if one falls, another will come after the killer and the target.
Some say that wyrths become one after killing the victim, or the victims. Some say they die.