Meet up with your group at Fort Snowhawk and then attack the Fort. To finish this quest, you must clear out all of the enemies. You will be regularly updated with how many enemies remain. When it hits zero, no more enemies will respawn and the quest will be completed but there may still be enemies remaining. They will likely run away at this point though. When you can, return to Ulfric to report it.
Unbound Before the Storm Bleak Falls Barrow Dragon Rising The Way of the Voice The Horn of Jurgen Windcaller A Blade in the Dark Diplomatic Immunity A Cornered Rat Alduin's Wall The Throat of the World Elder Knowledge Alduin's Bane Season Unending Fallen The World Eater's Eyrie Sovngarde Dragonslayer
Loss of Innocence With Friends Like These Destroy the Dark Brotherhood! Sanctuary Mourning Never Comes Whispers in the Dark Silence Has Been Broken Bound Until Death Breaching Security The Cure for Madness Recipe for Disaster To Kill an Emperor Death Incarnate Hail Sithis!