Nightmare
Nightmare can be found in the following locations: Pits of Inferno, Formorgar Mines, Cemetery Quarter, Edron (In multiple places during The Inquisition Quest), Alchemist Quarter, Vengoth Castle, Deeper Banuta, Krailos Ruins, Grounds of Deceit.
Nightmare has 2700 hitpoints. It is immune to earth, life drain, and death damage. It is strong against energy, fire, and ice damage. It is weak against holy damage. It can neither be summoned nor convinced.
Nightmare yields 1800 base experience points. Its loot can include gold coin, platinum coin, boots of haste, war axe, knight legs, ancient shield, power bolt, ham, mysterious voodoo skull, soul orb, death ring, demonic essence, skeleton decoration, flask of demonic blood, essence of a bad dream, and scythe leg.