Mountain Horror

How do I kill the Mountain Horror? I've tried all my weapons and spells without causing any damage.

You need the Mountain Hammer there.

Where do I find that?

It's lying on the ground in the far north west of the Strah Mountains behind a crackable red wall. The wall is a slightly different red than the walls near it. You need a maul to crack the wall. The maul is lying on the ground in the mid south. It can also be purchased in some weapon shops but the chance of its showing up is low. You'd have to sleep and sleep to see it show up in the inventory.