King of Hell (KoH) is the name of Zoro's techniques when he's using a lot of blackened haki, so much it creates a green aura around his swords, and ACoC together.
As we can see in the following scene just using ACoC, even with non-aura/normal blackened haki, is not a KoH technique.
Zoro has three sword serpent/dragon style technique which is when he uses even more ACoC.
There's so much ACoC it trails behind Zoro and looks like a serpent or dragon.
Zoro can use KoH and 3 sword Dragon Style together.