"Where have the vanished Command Spells gone now? Those existences that are after all the physical manifestations of magic, will most likely not disappear into thin air just like that, will they?"

"In theory, they should have returned to the Grail. The Command Seals themselves are things that the Grail bestows. People who have lost their eligibility as a Master because of the loss of their Servant, their Command Seals should be reclaimed back by the Grail. Yet, if there appears a Servant whose contract has been lifted because of the loss of his Master, the Grail will then redistribute the unused Command Seals it had previously reclaimed to new contractors."

Twenty-one engraved Command Seals distributed to seven Masters, disappearing after having been used once, and the Command Seals that at the end had not been consumed would then be amassed in the hands of the supervisor and commissioned for safekeeping.

- Fate/Zero: Act 9, Part 3