These creatures are either confined to, or they live in, smaller areas that are outside of the Realm.
The power levels of dungeon bosses vary, from Oryx (instant-death-shotgun level) to Malphas (god-level) to Dreadstump (Deathmage-level) to the Evil Chicken God (doesn’t-do-anything-level). Some dungeon bosses drop unique or very rare loot.