The Briarheart Geis is a unique Forsworn axe, having one more base damage point than Forsworn axes. It also contains a unique enchantment that does 5 extra points of damage to Nords.
This item can only be obtained by using the console command
player.additem 000AE087 1.
It can be upgraded with a steel ingot at a grindstone and benefits from the Steel Smithing perk, which doubles the improvement. It can also be disenchanted in order to learn the weapon's enchantment.