Maybe there were 4 Witch Gods; 1 of them was so ‘‘dark’’ that the other three joined forces to destroy him, but he was so strong that the other gods just managed to weaken him and maintained the triumvirate if this dark god ever managed to strike again.
Maybe this god spoke to Kymon and is now the entity that claims himself to be Empyrion, using the Kymon’s Chosen believe to increase his powers. Now that he is even stronger, he managed to drove the Witch Gods away, that’s where we enter trying to discover wth happened.
Most likely that latter, since it was mentioned by Mogdrogen that it is a name he haven’t heard for quite some time