The achievement is granted for winning a ranked multiplayer match using only "Khorne-themed units". This may not be automatically clear. To clarify, there are units which are always Khorne, Tzeench or Nurgle, and units which are "neutral" but may get upgrades associated with one of the Chaos Gods. Chaos Lord, Heretics, Bloodletters and Bloodcrushers are always safe. You can also use Tactical Marines, Havocs, Dreadnoughts and Predators as long as you do not choose Tzeench or Nurgle upgrades. The rest you just can't pick, as they are Tzeench-themed or Nurgle-themed by default.