In this quest you're supposed to get information about the whereabouts of a Elder Scroll.
The marker leads me to the College of Winterhold, The Arcaneum holder, Urug. He doesn't give the needed speakoption. (He's supposed to reach for some of his books and aid you or something)
I searched the net for some further information and this had something to do if doing the Mages Questline and/or the Septimus Deadra quest PRIOR to doing this quest.

This is a mainquest, so a huge gamebreaker.

PLZ help.

While not "Gamebreaking" since you can get the elder scroll ahead of time as mentioned in the Prima Guide by simply going to the dungeon. I haven't tested this out to see if it's true or not but I agree, it should be fixed. I didn't go and do the Dark brotherhood quests for the simple fact I heard if you did it before the MQ was over that you wouldn't be able to because of someone you needed to kill.

What dungeon ?
If it is the same dungeon to where the Septimus quest leads you then forget it.