As an extension to the old hermit idea. Sometimes, but rarely, the hermit, if he is human or elf, happens to be a wizard. Then, instead of digging a small room in the mountain, he quickly builds a tower (or another megaconstruction) on the top of a mountain and lives in it.
If he is a good wizard then he is harmless (if not provoked). He always helps in defending fortress and may heal dwarves, even such serious cases as loss of legs, hands or eyes. Оccasionally he produces enchanted items (very good swords, armor, etc), but for some reason keeps them only for himself, so the only way to obtain them is to kill him, but he really hard to kill.
If he is an evil wizard, then he is a human necromancer raising undead minions. He may be hostile, but as well he may not and will start a conversational screen with proposition of alliance - dwarves will supply him with corpses of their fallen comrades and of who attack your fortress, so he could raise more undead and would send some of his undead minions to help the dwarves to defeat their enemies. Rejecting his proposition will result in him becoming hostile and accepting it - war with all elven, some human and some dwarven civilizations. The wizard then starts behaving like a noble, sometimes making demands and there will be a several squads of undead at your side and you get all the loot from the hostiles they kill, because the wizard is interested only in corpses. Siding with him also may decrease dwarven mood for a few months or years until they get used to him and the new migrants will arrive being in an unhappy mood. The wizard also may help you to get rid of ghosts in your fortress and will sometimes kill your cats for his dark rituals. He is also a legendary bone carver, he cannot be controlled, but may create skull totems and other nasty things as a gesture of his good will.