Sever is a level 25 elite abomination mini-boss found in Shadowfang Keep during the first and second Scourge Invasion. He sits in a square room along the corridor that runs parallel to the Great Hall and Kitchen in Shadowfang Keep.
Sever is arguably harder than any of the other mini-bosses in Shadowfang Keep. He hits hard, is immune to roots, berserks at low life, and has a ton of life. Fortunately for groups that have a hard time on him, he is also completely optional and isn't along a route that is normally used by players going through the instance.
It is a good idea to bring him out of the room that he starts in and onto the ramp outside, as he uses Intimidating Shout, which can fear you into adds if you fight him in the close quarters of his initial room.