The Fire Guardian is a victim of the Torturers' experiments.
You will surely run into this fiery demonic monster on the second level of the Dungeon of the Torturers. The War Hammer and the Night Guest's Sickle are useless against the Guardian, but he is vulnerable to ice weapons. Be prepared for a long fight should you decide to use any other type of weapon against the Guardian except ice, as the monster is all but impervious to other types of damage, be they fire or physical attacks. The Fire Guardian is immune to many effects. He can't be poisoned, banished, or controlled. Cold is the only weapon that can harm him.