Inspired by the creatures from the Thomas Covenant books, Ur-Viles are other planar creatures that haunt the Underdark in the mountainous kingdom of the Dwarves. They lack eyes and are completely blind, but have no difficulty navigating dark tunnels. They always fight in a wedge-shaped formation that, from above, looks like bowling pins or a set of racked billiard balls, with the loremaster pointed towards the enemy.
Ur-Viles are potent spell-casters. They favor spells of darkness, fire, and acid offensively. They also use many transport based spells such as passwall, dimension door, spider climb, and distance distortion. Their loremasters carry the cast iron staves pictured below. Whichever individual holds the staff leads the pack, and if that one falls, another will pick up the stave. All of them also carry black cast iron spikes, which in their hands drip with an acidic secretion / spell that causes them to act as daggers of wounding. In another creature’s hands they are simply spikes.
The PCs referred to them as “devil bunnies,” for which they will never be forgiven.