This was a uneasy, and very brief, Alliance between the Chaos Space Marines under Lord Crull and Gorgut'z 'Ead 'Unter of the Orks on Lorn V. The two factions did not work alongside each other, and were known and seen to fight against each other, even if the enemy was around the corner. They did turn their attention to the enemy until they were beaten before resuming their infighting. The alliance was disolved when Gorgutz killed Crull during the final days of the war on Lorn V.
Chaos: Space marines, Tanks, Khorne Bezekers, Demons, Cultists,
Orks: Boyz, Boyz, Tanks, More Boyz, Sguigoths, And oh yeah, More boyz. Wit Big gunz.
The Battle for Lorn V