Gressil, Dawn of Ruin

Gressil, Dawn of Ruin is a one-handed sword with extremely large damage for its level. With its 257 max-damage, it gives rogues high hits/crits with instant abilities such as Riposte (old) and Sinister Strike.

Source
This item drops from Kel'Thuzad in Naxxramas.

Trivia
In Christian demonic beliefs, Gressil is the demon of impurity and uncleanliness. He is the third in the order of Thrones. He encourages one to comfort, luxury and indulgence.

Speculation
This weapon may have been one of the runeblades used by the death knights of the Scourge.