The Battle of Malrev IV was a battle in 1750 BBY during the New Sith Wars that saw the Jedi Order and Mandalorian mercenaries defeat the forces of the Dark Underlord at Malrev IV. Jedi Master Murrtaggh paid a group of Mandalorian mercenaries to make a diversionary attack against the Black Knights, minions of the Dark Underlord. As the battle raged, Murrtaggh entered the Underlord's fortress and killed him, although he fell to the dark side in doing it.
Sources
"The History of the Mandalorians: Exploring the Evolution of a Mighty Race of Warriors" — Star Wars Insider 80 (First pictured)
Evil Never Dies: The Sith Dynasties on Hyperspace (backup link) (original site is defunct)- The Essential Atlas (First identified as Battle of Malrev IV)