Damien and the Dark God would both be disallowed from interfering in the war actively. This meant that other than each other, they were not allowed to fight anyone from the other side.
A binding vow to the Heavens meant nothing to either of them. The Heavens could not contain them anymore, so those vows were meant to be broken.
Between them, the pact had to be placed into the folds of both Existence and Nonexistence, ensuring that they would lose everything if they chose to disobey.
Damien didn't know why the Dark God was catering to him so much.
This was not the same as the Saint Emperor in the slightest.
That man catered to Damien first because he dreamed of taking the Void Physique for himself and later because he understood that his role was meant for Damien from the start. The Saint Emperor ended up entrusting Damien with his legacy, as he raised him for that purpose even when he himself was unaware.