Well, OK. Point conceded - Phoenix wasn't forgotten - I spoke (typed?) too quickly. The point is it wasn't an extant primal before the Calamity... and while Griever is more than a little far-fetched of an idea, given the circumstances and purpose of the new primal's summoning I'm of the mind that it's at least reasonably plausible?

I mean, we got the Knights of the Round, arguably the most over-the-top summon in franchise history... nothing is really gonna surprise me at this point.

Of course, simply being a bastardized Rhalgr or something completely original isn't off the table. I'm still of the mind that a corrupted Rhalgr, hijacked by Ilberd, is more likely than Griever, but I must admit the possibility intrigues me.

... but, to get into the pseudo-science of it, without active followers how was Phoenix still around? His is a unique case, but without that answered...