The Six were the 3 most Dominant Fairies and Pearls that were captured and imprisoned by The Emperor in order to protect himself from being destroyed by them. The Six made a deal with The Champion of the God of Continuity, who made them a deal: he would grant them their freedom and immortality in order to kill The Emperor, but only if they forfeited their own lives after success. Furthermore, the Champion granted an extra clause: in case the Emperor died at someone else's hands, they could choose one of his kin to place their revenge on; they chose the Prince of Ravens. Foreseeing the Prince's death, the Blood Princess replicated the Prince's consciousness into a Shell, which housed one of The Six, therefore this Pearl was Suppressed, but because of the trade with the Champion, the Pearl did not die until the Prince did. After his death (at their hand) their own destruction arrived, and they all died.
The reason why the Blood Princess could not just revert the death was because if she did then The Six would come back.