Hellhammer is a Barbarian Unique Two-Handed Mace that buffs Upheaval. Like all Unique items they provide a powerful effect that can't be found on any other item. These effects can't be moved around like Legendary Aspects resulting in limited flexibility when incorporating them into your build.
How to Obtain - Random Drop
Rarity - Normal
Many Unique items change the way skills and builds work in significant ways. More info coming soon!