Before you start this quest, you must dismiss your pet. This quest will not allow you to control the Fel Reaver while you have an active pet.
Also, be ready for mobs to attack you and not your Fel Reaver. Once my Fel Reaver died and I was back into my character, I immediately had a bunch of demons on me. If this occurs, run up the ramp to the quest giver. It should de-aggro after a few feet.