Online:Stomping Sinmur

Quick Walkthrough
# (Optional) Gather the shards of Wuuthrad or form a group with other players.
# Defeat the Worm Cultists.
# Defeat Thallik Wormfather and Sinmur.
# Listen to Ysgramor.
# Talk to King Jorunn.
# Talk to Vigrod Wraithbane.
# Talk to King Jorunn.

Detailed Walkthrough
According to Vigrod Wraithbane, all that remains is to defeat the rest of the Worm Cultists, then you and King Jorunn will face Sinmur and Thallik. The battle would be too difficult if you had to face both at the same time, so Vigrod suggests separating them by doing great damage to Sinmur. You will need either the shards of   or mighty companions in order to do enough damage. 

The Shards of Wuuthrad 
The king brought the shard you retrieved into the ruin, but you will need the others. Vigrod says Thane Unnvald at Fullhelm Fort; you will have to ask around there to find it. The quest Approaching Thunder starts the Fullhelm Fort objective to retrieve the shard. 

There is another shard at Forelhost, although Vigrod doesn't tell you about it. The shard is where the dragon priest, Vosis, spawns. You need the quest A Walk Above the Clouds to get to the top of the mountain.

The third missing shard can be found in Lost Prospect by defeating Gullveig to complete the quest Those She Devours.

Valdam Andoren discovered a way to harness the power of Wuuthrad. Each shard has been placed in a runic brazier near the battle site. All you need to do is activate the brazier to unleash the power of Wuuthrad. 

The Final Battle
When you are ready, enter Trolhetta Summit through the door behind Vigrod and to the right of the Trolhetta Wayshrine. 

King Jorunn, Holgunn, Valdam Andoren and Walks-in-Ash prepare to attack the Worm Cultists. Help them defeat the enemies, then continue up into the Daedric ruins.

You should see Sinmur, a ghostly golden giant appear and walk down the steps. Even if you only have one shard, King Jorunn will call out to you, asking you to use the shard's power. You need to activate the brazier, which can be difficult while Sinmur is attacking you. However, once you activate it, Sinmur will stop attacking, which allows you to defeat Thallik Wormfather. Sinmur only starts to attack again after Thallik is defeated.

Victory

:''"Shor's Bones, the monster's down. Well done, my friends, well done. Kyne's breath! The Five Hundred Companions!"''—King Jorunn

Ysgramor and the companions' spirits appear to the west. You run down along with the other Pact leaders and listen to him address King Jorunn. 

:"You have united not just the Nords—the ancient elves and the crafty lizard-folk stand with us as well... A hard road lies ahead of you, but for now safe is victory... Companions, three cheers for King Jorunn and his champions!"—Ysgramor

Talk to the Skald-King. He is worried about Ysgramor's warning—what could it mean? He asks you to talk to Vigrod Wraithbane while he thinks.

You tell Vigrod outside that Sinmur is dead, and he tells you the Rift is finally safe.

Talk to the king again to complete the quest and receive the Mace of the Companions, plus a skill point and gold. He will return to Mournhold and hopes to see you there as the Pact plans to take the fight to Coldharbour.

Notes

 

Quest Stages