I have a theory that perhaps the sage himself made a contract with the shinigami, seeing as there were many masks, that would imply many shinigami, no? What if this "contract" involved sealing the sage away, by his own will, with the only way for him to be unsealed is when you bring the specific shinigami mask (guarded by the Uzumaki, the clan of one son) to the Uchiha tablet (Guarded by the Uchiha, the clan of the other son), and perform the ritual that Orochimaru has just performed. Perhaps this was done so that one day, if ever needed, the clans of his two sons could call upon him for help. Just a thought.
