The sword can be found in a building in Blackreach and is equal in power to an Elven Greatsword, but swings a little faster. It has a very strong enchantment which consists of...
Poison Damage Extra damage to undead with a chance of creating a turn undead explosion High damage on critical hits And a chance to heal the wielder by 100 points if the enemy is killed by a critical hit.
It can be tempered with an empty greater soul gem if you have the Arcane Blacksmith Perk.