I'm supposed to kill a troll shaman, but it is not there. It's not in the troll mound or, as far as I can tell, anywhere else.

If you start at the cave/grove/stronghold proper, head towards the spot where you enter the map. Or you could leave the map and head to, say, Trademeet, and then enter and head to the cave... She's near the entrance, to the right of the mound. If she's not there, memorize and cast Detect Evil. If she's not there, you have a problem.

Seems I was looking in the wrong place. Thanks.