Then something happened,” Baljos said. “The notes are not exactly clear, but it seems like they found and took on a partner, another Dark Jedi, and at some point Irek and the new partner got in a dispute and dueled. The partner was killed, and Irek took a lightsaber thrust right through the skull. He died.”
--Enemy Lines II: Rebel Stand

And with the shadow nerve network of meltmassif lacing his body, he had a fundamental connection to the Dark that rivaled Cronal's own.
―Star Wars: Luke Skywalker and the Shadows of Mindor