This is actually explained in the DLC, Chakravartin (God) was bringing both Asura and Vlitra back. Applies to Vlitra as well, since it's never explained how it keeps coming back. First when thrown from an orbiting spaceship and second after being tossed into lava with both arms ripped off. The sequel explains it by his armour blunting the attack. Then he wakes and stands up, completely shaking it off. In Assassin's Creed II, Ezio is stabbed in the stomach by Rodrigo Borgia in the penultimate boss battle and collapses in a spreading pool of blood, complete with fade to black.It's possible that the Ritual Stone had something to do with it, but it's never specified in-game. She appears at the end of the fight, no worse for wear with no explanation given for her recovery. In Ashen Amara is seemingly killed off in a very dramatic fashion during the boss fight with the Shadow of the Ashen.
