The above quest is unfinishable since Christopher Varkaant (the npc your supposed to speak to) is inside an area that is unreachable at Delphe Castle, i've been going around everywhere in the area looking for a door or ladder up to where he is (while dealing with harpies) even tried to use teleport (wizard char), to no avail, there is no way to reach him, i can see him from another part of the building but that's all.

Nvm i eventually found the stairs upto the upper floor, they are in archway on the left before you come to the southern watchtower building.