You mean where he goes from being merely the age of the cosmos to several times longer that all of pataphysical existence? Ehh... depends on when it happens. Like, if it's in the second act, that's fine. That's just like, a step in the maturation cycle. Sort of how they handle it in the Godfather. Remember that part where Tony gained the ring of power and fell under Saruon's will? I mean, that was totally telegraphed, but still very cool. Putting that on the cover wasn't a spoiler. But if it happens after the climatic ending in the epilogue like say... Any given episode of Cheers where Norm devours someone's soul and makes everyone watch (Yeah, I know it's a comedy, but the writing principle is the same), then yeah, that'd be a spoiler.