Godslayer explained in vague terms what needed to be done—vague only because he himself wasn't sure about the exact process.
He understood one half better than the other. The merging of two spirits was something he knew how to do from his memories. Apparently, the more developed the spirits were, the harder the chances of them merging.
Given the current situation of the two spirits—Godslayer being weakened to a point where he wasn't even sure if he could retain his intelligence, and Midnight, who had lost his intelligence—the chances of success were quite high.
Even if there was a failure, there was no danger to Midnight himself, as Godslayer was going to expend all of himself in the attempt. Even if he failed, Midnight would still end up stronger in the end.
On the other hand, the task of merging a soul into an Origin was something completely outside the wheelhouse of even the Artifact God. He had never in his life heard of something similar being done.
