Defeating the boss; Morokei in the Labyrinthian dungeon for the quest to obtain the Staff of Magnus.
Morokei drops the Staff of Magnus once defeated.
I struggled as im sure many of you did for a long time on this guy, he just killed me in one or two lightning hits and it was ugly but if you get right up on him at the start of the fight and start shield bashing him or heavy attacking it will delay his casting. If you see him about to cast something, use slow time shout if you have it and shield bash or power swing and it will stagger him out of the cast, should buy you more time to kill him. IF youre really in a bad spot you can ignore him altogether and climb the stairs and find an exit in the back of the room and just come back later when you are stronger.