SideQuest - The Nook – Defeat Bheitnir
First head to the courtyard. Find the scalable wall with a deathgrip hoop in the southwest corner of the courtyard. Blow up the crystal to reveal a Book of the Dead page (11).
Return to the Nook's fiery chamber and use the deathgrip to reach the Boatman Coin (40). Deathgrip again to reach the narrow path and wait for Bheitnir to fly by. You'll know that you're ready to battle him when the music changes.
Since you don't have that much room to manuever, you can use the hand-holds on the sides to avoid the boss' damaging fire attacks. You can exploit the fact that you won't die by deliberately falling to the lava below. Bhietnir will cease its attacks and fly away as you recover from the fall. The good thing is that its HP won't reset or regenerate; you can chip away its HP until you kill it.
Use fast attacks like Teleport Slash and use your Reaper Form if necessary. Items will be much of use here so don't hesitate using them like crazy. Return to Thane to complete the quest.