Due to what may be an oversight these are not randomly generated. They will usually only be found in Grund's Stronghold, where up to two may be generated. They can appear elsewhere due to polymorph, and also in aligned levels and branches of the dungeon.
They don't have enough hit dice to cast the more threatening spells, but are quite capable of cursing your items. You may therefore want to kill them at range, particularly if you lack magic resistance.