The Diablo Armament was a "simulation" of Diabolos created by the Allagans somehow recording his soul's wavelengths from when he was summoned in their era then using that data to transform a dragon into him.

Still, given what we know about how the soul and body share a bond, I feel like it's possible that the voidsent who are summoned through vessels may simply end up having their souls yanked back into their real bodies in the 13th if they "die" on the Source. I'm honestly expecting to see some familiar faces in whatever dungeon we're liable to get in 6.2.