You recovered the doctor's tome, but you didn't finish him off using the flesh giant?
Get back in there and kill him before it's too late!
Completion
You discovered and put an end to the doctor's machinations.
<The banshee-controlled witch smiles and places a large hand gently upon your shoulder.
That's a fine poke in the eye for the Lich King, but I think we can do better.
Notes
After killing Osterkilgr, take a left and mount the flesh giant. #1 is melee attack on a single target, #2 is AoE knockback and dmg, #3 is ground stomp (AoE dmg), and #4 is single-target pull. Don't waste your energy using #1 on the group mobs; use 2 and 3 on them, keeping them away and killing them until Dr. Terrible appears. As soon as the good doctor spawns, use #1 on him, knocking all of the other enemies away with #2. As soon as the doctor is defeated, the rest of the room will despawn.
When the doctor reaches 35% health, he will begin to cast a 4 second heal. He will do this regularly, so make sure to save your interrupts for the heal.
Dr. Terrible yells: WHO DARES?! I AM GOING TO FLAY THE FLESH FROM YOUR BONES EVEN IF I HAVE TO DESTROY NERGELD TO GET TO YOU!
Upon defeat:
Dr. Terrible yells: I'LL RISE AGAIN AND WHEN I DO I WILL HAVE MY REVENGE!