Alright reading more closely now I will amend what I said:
The first war: the Sun God fought a war with the Earth God, the Earth God unleashed the hell flame which killed everyone. At some point the earth becomes flooded?
The second war: The Sun God wants to fight the Earth God again? There is a Forest God and a Sea God (Imu?) also? Anyway the twenty kings all gang up on Nika and kill him because they don’t want to unleash the Earth God, they just want it to stay buried.
War 3: This is when the Sun God will actually kill the Earth God.
So the Earth God is the final OP villain, whatever the hell that is. The WG exists to prevent the Earth God from waking up and keep it buried under the ocean (remember the true OP world is flooded).
So basically the Earth God is the final villain of One Piece. Imu is a subverted final villain, the Earth God is far worse.