Souls
Vangel, possessing knowledge in the art of alchemy or having access to ancient texts, could have attempted to recreate Tenebres' blood using other ingredients and alchemical processes. By carefully studying the properties and composition of the purified blood, he might have devised a way to replicate it to some extent.
Before he could confirm one of those, Aion was forced to return… more rifts were appearing outside, and he could tell that Mareia was just trying to make him waste time with that.
In any case, Aion always tried to avoid making his weapons absorb the bodies of humans since he didn't want to process them. Even with his enemies, he would feel grossed out. Still, to think that such a thing was possible… to unlock classes by making his weapon absorb them without processing it.
"Wait… I thought that Vangel's blood was what made me feel bad, but after doing the same with the bodies, nothing changed…." Aion furrowed his eyebrows.