- Receive the quest by walking into Valthume and speaking to the ghost.
- Collect the Vessels from each of Hevnoraak’s minions. Each is guarded by a Draugr of some kind.
- Kill Hevnoraak with the help of the quest giver.
After receiving the quest, head for the room with two turns that is filled with oil and pressure plates. There will be a retractable stone wall on the left side and a handle just to the left of it. Hit handle the handle and pass through. This will lead you to the first vessel, guarded by some variety of Draugr.
Turn around and continue through the dungeon until you find the Valthume Catacombs. Follow the quest marker to the gate. Just before the gate on the right will be a chain which opens it. Kill the Draugr and collect the Vessel.
Continue on and pick up the Iron Claw. The order, top to bottom, is: Dragon, Eagle, Wolf. Pass through the door and kill the Draugr guarding it for the third Vessel.
Return to Valthume and speak to Valdar. He will tell you to empty the Vessels into the sconce. Do so and sit on the throne in front of the sconce, triggering the battle.
Watch the lightning go, then kill Hevnoraak and loot his mask. He is one of the eight powerful dragon priests in Skyrim.
It is advisable to enter this quest well-armed, with a high-level character and lots of potions. If you have a companion accompanying you and you want them to make it out alive, it may even be a good idea to tell them to wait in another area while you battle the tougher enemies alone. At level 16 you can expect a few rooms to contain multiple high-level Draugr at once, such as Deathlords and Scourges.
There is also an Aura Whisper word wall and a major loot chest in this dungeon, accessible only after Hevnoraak has been vanquished. Go back through the door to the Catacombs after you kill him and the bars will have receded into the ground, allowing access to the previously inaccessible goodies.
It is possible to obtain the word of power without even fighting Hevnoraak if the vessel immediately in front of the word wall is only the second that you find. In that case the bars will recede when you pick up the second vessel. You may then proceed to find the last vessel and challenge Hevnoraak at your leisure.