He broke a deal with the devil and got away with it
Technically a devil - Kil'jaeden, to be precise. The Eredar lord of the Burning Legion twice commanded Illidan to kill the Lich King and both times our demon hunter came up short. After his loss to Arthas, Illidan retreated to Outland and began fortifying against the Legion, who would be coming to punish him for his failure. We don't know what would have eventually happened since he was killed by adventurers (read: us), but he apparently got off ok as far as the Legion was concerned. I'd call it a win. The poor guy could certainly use one.
Published Aug. 8th 2015