After you get the book you need and do the report, you have to fight a guy in the Labyrinth that will give you Orthrus' collar. My friend found himself unable to go in the tower by door or port to me. Is this a solo quest because it looks like it is going to be a multi level tower?. Can other people who have the same quest help you? (I dont know any though) :D
I think maybe you have to have the same quest and belong to the same school. I was able to do the level 18 quest for my pyromancer with a friend that was also a pyromancer and had the quest. I am not sure about the higher level quests though.