Revived Vampires were vampires that had been killed and whose souls had adapted to the Spectral Realm becoming Vampire wraiths. If they were then able to re-inhabit their corpse they would bring their spectral advantages with them and become Revived vampires.
A Revived Vampire was created when the spirit of a Vampire Wraith was allowed to re-enter its corpse and once again wear flesh. This occurred because the unfettering of a vampire's soul was not enough to kill it truly as the flesh remained behind even after the creature's 'death'; and if the method of a Vampire's fatality was removed the soul of the dead Vampire would come hurtling back to its body and reanimate it, becoming a Revived Vampire. This usually occurred when Raziel removed the weapon that still impaled the Vampire, but in Ash Village and also in the City it is shown that even Vampires who were killed via submersion in water will revive if the water is drained from the environment, it has been theorised that only strong sunlight can destroy a Vampire's remains to such a degree that reanimation is impossible.
When the Vampire Wraith returns to its material body it brings with it the abilities it inherited in the Spectral Realm; most notably the ability to siphon of Soul Energy from a small distance away. As with the Spectral Wraiths, a single hit is all that is needed to establish contact with Raziel and the Revived Vampire could then begin draining him of energy until Raziel broke the contact by counter-attacking or retreating a distance away.
Raziel encountered Revived Vampires whilst traversing Kain's empire in search for his sire and his brethren. The first time Raziel came face-to-face with a Revived Vampire was in the Necropolis; and was of Melchiah's clan. However the most notable Revived Vampires were the Tomb Guardian and Dumah.
Revived vampires are not present in the alpha builds of Legacy of Kain: Soul Reaver and notable vampires that have are revived in retail are not treated as such in the alphas: Dumah for example is static with no effect and the Tomb Guardian is treated as a regular vampire. One exception to this is Zephon: although not a revived vampire in the retail game Zephon is given the same sparkle effect and can use the Draining Leash to take soul energy from a distance - that ability is also given a sparkle effect rather than the energy tether seen in retail.