So I've got the horn, and I'm in the ruins. My questlog says to blow it, but I can't. I press "E", and find it under Invoke, but when I click it nothing happens...
i had problems with this as well. the funny thing is you dont blow it you place it on the pedestal closest to the back wall good luck and kick some butt!
you do not need to invoke the horn, just place it to pedestal and talk to wizards image and take the relics boons.
Then you must return to Emindor and make the gates of Fargrove reopen again.
Wish you good luck