Pupating, also referred to as evolution or devolution, is a characteristic unique to the Vampires descended from Kain. All such Vampires evolve by "retreating into quiescent states from which they emerge transformed", developing new dark gifts and abilities corresponding to their individual traits.
The pupation process in Kain's descendants was a consequence of the Corruption of the Pillars, initiated by Nupraptor, which corrupted Kain's soul at birth. When Kain reanimated the Sarafan Warrior Inquisitors to serve as his Lieutenants, their souls inherited this corruption, and their bodies began to deform and devolve over time as a result. Highly evolved Vampires were referred to as Adults, while their younger counterparts were known as Fledglings. Fortunately, Raziel, having been executed beforehand and resurrected as a wraith, was spared this fate.
Vampiric pupation and pupation grounds differed depending on the brood: Vampires of the Melchahim clan burrowed underground in the Necropolis, whereas the Zephonim and the Rahabim cocooned themselves in the Silenced Cathedral and the Drowned Abbey respectively.
The background details of cancelled spin-off Nosgoth offered a little more on the processes of pupation, mentioning that each vampire would gorge on Blood before entering a pupation state - which would sustain them during the process. After emerging they would be disorientated and ravenous, with vampire 'midwives' on hand to tend to their needs as they recovered.[NOS-Official]