So, I've been thinking - it's pretty much common knowledge that Undead in general are Super Effective against the living, and Not Very Effective against other Undead - you generally want some Positive Energy based stuff to fight them.
Yet at least one type of Undead, Boneyard from Libris Mortis, is perfectly capable of reducing any bone-containing creature into a boneless corpse - even other Undead creatures.
This made me wonder, is it the only example of such Undead-Hunting Undead creature, or are there others like it? Can a Necromancer create a buddy who would protect him against, say, other Necromancers and their minions?