Engra Deathsword, during the Great War of Chaos. This towering Norsecan Chaos Champion was a general in the armies of Asavar Kul and led the extermination of the the city of Praag. His sheer brutality was SO great that he caused the very city to mutate and those mutations wailed in agony as the saw the horrific punishments Engra meted out against those who dared challenge him. Engra is fucking proof that you can have a Finnish woman name and still be completely badassed.
Archaeon claiming the six treasures of Chaos.
The saga of the Gorehunt tribe, which was given in the 7th edition Chaos codex by Phil Kelly. The Gorehunt were a tribe of Norsemen dedicated to Khorne who vowed to travel to distant lands and offer up the skulls of enemies far beyond to their god. So they went on their longships and sailed to Araby and, to use the vernacular, fucked shit up. The Emirs had to send armies of thousands, and hundreds of thousands to defeat the blood crazed Norse, and all those armies who absolutely slaughtered. Now, this seems awesome by itself, but keep in mind that the Norscans were only a hundred in number. These champions of Khorne were annihilating armies several times their numbers. The Norscans carved through the armies with such abandon that a raging stream of blood began to flow through Araby and still does. Even arcane creatures couldn't stop the warriors of Khorne; and simply caused their killing frenzy to grow. Finally, the Emirs sent, as the story put it — "gigantic beasts of war, and armies that hid the dunes with their number", and the Norse's last stand was a glorious one. And the river became larger and larger, and though the Northmen were eventually killed, they had defeated armies that dwarfed them utterly and pleased their master, for the crimson river still flows through Araby in testament to their skill and sacrifice. Seriously, there hasn't been another story that truly encompasses the badassery of Khorne as this one does, Phil couldn't have chosen a better one to put in the codex.
Grimgor Ironhide defeating Archaeon, the single strongest warrior of Chaos, in single combat.