Khârn the Betrayer is a character from Games Workshop's Warhammer 40,000 universe. He is a Chaos Space Marine, possibly one of the most fierce close-combat opponents you could ever face, and utterly dedicated to killing as many people - be they his opponents or his allies - as possible, all in the name of the Blood God, Khorne. He is a Khorne Berzerker by definition - which already sets him apart as being extremely handy in a melee - but, with skills that go above and beyond the norm, and with a very nice chainaxe, he borders on the unstoppable.
"Kill! Maim! Burn!" - Khârn
Khârn has been a member of the World Eaters legion of Chaos Space Marines since before they turned traitorous in the Horus Heresy. Indeed, it was during the mass rebellion of formally loyal superhuman warriors that he died, just as the Emperor of Mankind slew the Warmaster Horus. As they retreated, the remnants of the World Eaters found and recovered his corpse, which afterwards was revived - more than likely by the Blood God himself. Since then, he has continued to slay in his master's name, never having since been bested in combat.
As his title of 'Betrayer' suggests, he has absolutely no qualms about killing his own troops, if it means that blood is shed for Khorne. It was his actions on the planet Skalathrax, when forces he was a member of were fighting fellow traitor legion the World Eaters, that gave him this name, and ever since his own legion have been more than a little wary of following him. This doesn't bother Khârn one bit; all that matters to him is killing in the name of Khorne.
Being a favoured champion of Khorne, Khârn bears the Blood God's mark. He also wears Chaos armour, and weilds a plasma pistol and an ancient (and suitably huge) chainaxe, 'Gorechild'. Quite frankly, he is a monster, and if he happens to be leading a squad of Berzerkers, almost invincible.
All information written in my own words, sourced from Games Workshop's Warhammer 40,000 and Codex: Chaos Space Marines books.