// Depending on how you look at it Sephiroth is the Nyarly of his world or greater. After the whole thing with Jenova he would be a kind of Outer God. At the very least he's the subordinate of one//
Jenova has the modus operandi of a Great Old One, and at the end of Advent Children, Sephiroth expresses the desire to be like that.
But he is somewhat of an antichrist figure, which does align with Nyarly. But in the Cthulhu mythos, Nyarly is not a Satan figure, it’s more that Satan is a Nyarly figure distorted through a human point of view.
I’d think that Nyarly would be like something foreign coming to challenge Sephiroth’s power and even make claims about being Jenova’s superior even though they’re not from the same canon/world.