Is there a goal here besides mindless thrashing skeletons and rats ? I think i've cleared the entire island, including everything inside and underneath the keep, then i freed a Khajit who got stuck there and had been living off mushrooms ever since. When i speak to him, all he says is "thank you for freeing me, lead the way". But where to ? I've been running around the island searching for more skeletons, but there are none left afaik.

So when does this quest officially end ?

It ends when you kill all the skeleton plus the lich. Basically just a hack and slash quest.

I haven't seen a lich yet, where can i find him ?

Deep inside the castle, it's very dark so you'll need a light source ( or nighteye ) to see your way.

Found the lich, there was one room i overlooked. The lich was a pretty tough fight and i had to retreat to restore my health several times, but in the end i finally killed him. I went back up to the Khajit, thinking that the mission had finally ended, but no such luck, he still had the same lines for me.

I guess that means that somewhere somehow a lone skeleton must have escaped my attention, but i don't have the patience to stripsearch the whole damn place all over again. The lich is dead and the Khajit can kiss my ass. :D

Always a good attitude

If you think you killed everything. Go back to custom and excise office in seyda need and talk to the imperial legion guy ( captain something ) for your " excellent " reward. :D

did you get the shield?

There's a shield involved ? I haven't gone back to Seyda Neen yet, i guess i'll do that once i'm done becoming Hortator of the three houses. Maybe then i'll finally get a message that the quest is completed succesfully.