Black Castle creatures

From Glen Cook Wiki

The strange denizens of the Black Castle, working to complete the gate and free the Dominator from the Barrowland. There are several of them, some of whom seem to be identical to one another. They wear black pantaloons and black hooded shirts; their faces are "sharp angles and shadows, lustrous, olive, cold, with a pair of softly luminous eyes." They have some magical abilities and are very hard to injure with conventional weapons.

The nature of these creatures is never explained, but the fact that they seem to appear within the Castle (and the other Castles seeded far away) indicates that they are summoned creatures of some sort, demons conjured by the Dominator's sorcery.