All should be revealed soon, but I did suddenly have a suspicion that Calyx is one of our missing four Ascians. It is possible we will get a trio of them as the antagonists of 8.0. Yes, we saw Altima and Deudalaphon's masks in Gaius' possession, but that doesn't mean they are "dead". Calyx could possibly be Pashtarot's shard. This could explain his knowledge of primal summoning and his cold and calculating personality. That "ascian" we saw could be Altima, Deudalaphon, or even Halmarut.

But I still think Calyx is definitely dying in 7.4