There is a couple of bugs in this quest. Once you collect all the items it gives a wrong and confusing entry in the journal:

The description is wrong for two things:
-The Imp is not in the Temple of Bane
-He is not in Cloakwood Forest but in Sharp Tooth.
The other bug is that when you kill a Minotaur the journal updates and consider you have a minotaur horn. However, if you don't actually have the item, the imp doesn't give you the option to complete the quest.
Since minotaur's horns don't drop with every minotaur, this make it confusing and one may go back thinking that he/she already have the horn.
Cheers!