The Mirror of Erised

Troll

Answer: OTTOMAN

Follow the instructions. Not all the instructions are equally important, but it is a troll puzzle, after all. Upon doing all this, you should arrive on the bridge connecting Howard and Platt Halls. Since you are meant to go and find the troll, and trolls live under bridges, you need to go back outside and under the bridge. Taped to the bottom of the bridge is the answer.