Interesting Question Regarding Voidsent.....

[KageTokage]

I think it was implied that you can't truly kill voidsent unless they bring their physical bodies over to the Source, as their soul is simply banished back to their original body back in the void should their vessel be destroyed.

That said, I feel somewhat doubtful we've seen the last of some voidsent like Diabolos because while did he did get blown up by the Nullstone, it was stated that it nullifies pacts by destroying the vessel involved, not necessarily the soul occupying it.