I think Krosis was the first one I found, actually. Near the end of the Thieves Guild quest line, there's a mission that takes you into a Dwarven ruin up in some mountains. Before entering the ruins, look around a little bit. not far from the entrance You'll find a dragon and a word of power wall thing. When you get near the wall, Krosis will pop out of a coffin. Just be careful, or you might end up fighting him and the dragon at the same time!