I gave it some thought based on rewatching some scenes and I suppose kai could have entered oblivion, meaning he is permanently dead as his soul is destroyed. Its important to set that apart from malivore, its not the same thing.
It is also important to note this doesnt mean kai cant come back, however if his soul is entirely gone, it would mean he'd have to be remade and i cant see anyone doing that, the ability to create life and the soul, a villian like that could come eventually.
Im not going to take a guess at which villians could have also entered oblivion, cause whomever sends souls to which place, they are wishy washy, it depends on what sin means to the one who decides the fate of the souls. Cause for example you had vicki go to hell and she only became bad cause of being a vampire, whereas dahlia immediately went to peace when she died even though she wanted to cheat the natural method and even killed and kidnapped children.
So god only knows where kai is at right now.