A young drow got 'left behind' after a raid to the surface. He is a mid-level fighter, slightly lower-level magic user. Maybe give him a few pet large spiders for some extra challenge. He could take over a farm house (or two) with charm spells (maybe even charm a few of the animals). He could try and trick the party into finding the entrance to the drow realm for him (or maybe kill some inconvenient big thing). Anyway, as there's only one drow, a party of four or five lower level characters wouldn't really be in too great a danger.