Crunch
There was no greater enemy of people than their selves. The Elven Ruler was no different, and it has been proven countless times before. Therefore, Nikolai needs to use her as a distraction as he sneaks into Celestia and steals Val's powers.
For every god, whether ancient or celestial, there is a medium for their powers. That is why there was a chance to steal their powers, as long as the gods were weakened or distracted.
Nikolai was confident about his ability to obtain Val's inheritance, but there was minimal time left before the coup, the apocalypse, and the beginning of his suffering.
He wanted the power to rule over the dead, one that the gods dreaded.
Even though Val was supposedly a vile god, hated by the other celestial beings, he was still a part of Celestia. Therefore, when the gods descend, they will declare that Celestia protected all.