Just a note to add to the solution post by vursus.
In one of the flying fortress we will meet Xanlosh, a nigromancer who is a part of the quest Jonelath (Our nigromancer if we choose him in Sentinel island) gives to us.
HE is not the commander of that fortress, the commander is named Stone, and it's located just outside the location we fight Xanlosh.
If you think my solution can be better say it in comments so I can improve it. Thanks ;)