Command Spells are given out by the Grail at the beginning of the war. Mediators in this war receive the command spells that went unused during the previous one. Therefore, the most common and likely way to receive new command spells is through the Grail, one way or the other. If a Master loses all their spells, there is a high probability the Grail will re-select them, and thus give them three new spells. If a Master completes a task given by the mediator, it is very likely they wil be offered command spells in exchange for completion.
Admittedly, this is likely not the method you have in mind. To transfer spells between people, either the mediator or the Master must make a contract, and thus entrust their spells to someone else.
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Command Spells are given out by the Grail at the beginning of the war. Mediators in this war receive the command spells that went unused during the previous one. Therefore, the most common and likely way to receive new command spells is through the Grail, one way or the other. If a Master loses all their spells, there is a high probability the Grail will re-select them, and thus give them three new spells. If a Master completes a task given by the mediator, it is very likely they wil be offered command spells in exchange for completion.
Admittedly, this is likely not the method you have in mind. To transfer spells between people, either the mediator or the Master must make a contract, and thus entrust their spells to someone else.