What if Rimuru was instead reborn as a primordial demon; the strongest in combat and intelligence. He reincarnated just before Star king dragon Veldanava created the omniverse; it was nothing but a void, his soul being exposed just a little to turn null made his soul undergo a change. Making him an individual with a high evolutionary potential

Veldanava, even as a God, was hard pressed to control the energy of 'nothingness': the origin of all his creations, [TURN NULL]. The actual reason was his finite stomach. So to solve this one, he decided to create an entity suitable to control his powers for him. This leads to the birth of the first 'True Ancestor', the first "True Demon." Later to be known as the Mother Progenitor.