That's Tuska Daemon-Killa, the only warboss that is directly fighting Chaos, having led a WAAAGH! into the Eye of Terror because he knew where da best fightin woz. Thankfully, Orks don't run on normal lines of thinking so the Warp didn't make their heads explode. He died killing a Daemon Prince in battle and the WAAAGH! devastated dozens of worlds in the Warp before losing steam, and Orks being Orks the Chaos are still fighting infestation on those worlds.
Also, Khorne thought Tuska was so amazing for the sheer balls of what he did, that he resurrected him and his army so that they could die every day in battle against Khorne's armies. Tuska is ok with this.