A 30ish level monster in
EverQuest. I have battled Shadow Man
Warriors and Shadow Man
Necromancers, and there are probably other classes of shadow men as well.
They are invisible, but can be detected by the weapon or item they are carrying (the Warrior carries a sword and the Necromancer carries a spellbook).
Several quests involve slaying shadow men. The items they drop are no rent, which means the quest must be completed without logging off.