When Voidsent Die

So maybe some spoilers here from the Sky Pirate raids etc, but not anything from any recent content so I think we're good.

So there are some voidsent like Diabolos, and Ferdiad that we've fought more than once. So my question more or less is...what happens when a Voidsent is slain in Eorzea/Hydalyn? With the exception of some Voidsent like Diabolos being perma killed with the Nullstone, do most of the voidsent we slay just go right back to the Void, similar to how WoW demons would be sent back to the Twisting Nether to regenerate their bodies when killed under normal circumstances? Or is it just more powerful voidsent like Diabolos that would require special means to kill?

I'm not 100% on it myself, to be honest. The hints we have seem to conflict.

Often, voidsent are spoken of as if there is no life or death, only an anomalous essence incompatible with life or death as we know it; which comes from the void, manifests here, and is driven back unto the void by the destruction of its vessel. Supporting this is the lore book entre on the Nullstone, which says it was used to destroy a voidsent's vessel and nullify any pacts made with it, severing all ties to the corporeal realm.

But sometimes voidsent are spoken of as "ending." In 2.00, we had a guildhest where we would slay a voidsent and their "essence" would be left as flaming residue rejected by our Lifestream which another voidsent would consume. We saw similar behavior from Diabolos when we slew Scathach. Supporting this is the in-quest text for the Nullstone, which says it "disperses the essence of even the most powerful voidsent, obliterating it completely."

One suggests we annihilated Diabolos for good, one suggests we merely sent the newly crowned void king back where he belongs. I'm not sure what the solution is. Perhaps there is a moment of vulnerability when a voidsent is driven from its vessel and exhausted whereby it can be consumed, but otherwise it will seep back into the void and regain its senses, I dunno.

I don't like to think that Diabolos is done and dusted, however. How often do you see the rise of Diabolos? One more rematch, says I. LV99.

From what little I recall, voidsent are typically summoned into avatars, corpses and the like, so killing those doesn't actually kill the actual voidsent. In all likelihood, yes, they are sent back to the Void/13th Shard or are stuck in the Source until they possess another suitable avatar.

This, of course, infers that we actually bother to double tap the voidsent in question. More often than not, they seem to just play dead or get weakened enough to retreat and fake death.