Arceus is a fake deity, a Demiurge if you will.
Don't fall for the lies of petty Yaldabaoth, know that there is a higher power in the Pokeverse waiting to reveal itself. The true creator, the architect, the kingly one, the axiom.
Ultra Necrozma is but a disciple of the true highest power in the Pokeverse, sent to bestow the masses with the light of truth and free the souls of humanity from the tyranny of Arceus the vile.