Gino at Garm's Rise wants you to plant the Hardpacked Explosive Bundle near Frostgut's Altar, kill Tormar Frostgut, and then escape the cave before it collapses on you.
Hey there! Joining us for the assault I assume?
I've got a special task for you... we're going to bring the whole cave down on their heads! Towards the back of their cave, you should find Frostgut's Altar. Sneak up and plant this hardpacked explosive bundle somewhere close to it.
Once the charge is placed, stick around just long enough to make sure Frostgut bites it, then get out quickly! There's no knowing how long the cave will last once that thing goes off!