I don't, actually. Epic magic is something I'm terribly inexperienced with. All I know is that it's theoretically the most powerful spell able to be cast in my setting as it was the product of a divnity-draining ritual performed by all of the Elder Lords (Sidereals one and all, about 15 in total) and was powerful enough to blast Immortal Darkness from the Pinnacle of Creation for all time, though insufficient to kill it. (As Baalzoth has Transmortality, but more for story purposes, really).
I really have no clue what it would even do, it's more theoretical. I imagine it would be able to hurt Khorvanis (or even Avinion) bad, so it's likely on par with an Omnific Abilityh or two (or three) in terms of raw strength. Maybe some obnoxious number of d1000s powered by something like the Oblivion ability? That would make it cause grievous wounds even to Eternal beings. Maybe some sort of power to blast through any kind of barrier, ignore miss chance and the like. I can't see it having a spellcraft of less than 10,000 though. We're talking something like 1,000 Divine Ranks converted into raw, one-time-use energy.
After all, the mass suicide of the entire Sidereal pantheon should have some pretty spectacular backlash.
Edit- I should probably mention as well that since it was already cast, Khorvanis is technically immune to ALL magic from his universe, since there's literally nothing left that could throw that much power around. Eriahim could have, once, but he's horribly maimed and can't call forth even a fraction of the might he once possessed.
I suppose Khorvanis himself could, if need be, by sacrifing a ton of his own power, but magic isn't really his thing. (That's conceptually, too. No way he could hit a Spellcraft that high.)