Apolyon, the Soul-Render

Apolyon, the Soul-Render is an epic two-handed sword for arms warriors and retribution paladins. This weapon was the highest DPS melee weapon in the game, prior to the release of Wrath of the Lich King, displacing the previous Cataclysm's Edge for 1st place.

Source
This item drops from Kil'jaeden in Sunwell Plateau.