As much as Eos wanted to stop this power from entering Existence, he could not; it was the rule of the game, and without it, the situation for all life would become bleak.
Just because Eos and the Painter were playing this game did not mean they could not physically fight each other, but there was a saying in one of the mortal worlds deep inside the Origin Tree, which goes, "When two elephants fight, it is the grass that suffers."
If he were to battle the Painter, then the entirety of the Grand Void would become a place of devastation, and the Origin Tree would survive it, but all of its inhabitants would be dead, even the Primordials.
The only way to fight the Painter was via this board, and that meant he had to follow the rules and counter the moves of the Painter with his own.
And so he could only watch the influence of Drift enter his Origin Tree.
