The Command Spells, or Command Seals, symbolize (and, when used, enforce) the absolute authority of a magus over a Servant, and are the holy marks that appear on the body of the magi who become Masters for the Holy Grail War. The system was created by Zouken Makiri.
The shape of the seals is different for each Master; they reflect the magical characteristic of the magus. The Command Spells of the magi from the Tohsaka family have the general form of a circle, in reference to “harmony” or “wisdom”, while Kiritsugu Emiya, Shirou Emiya and Waver Velvet have a sword. The Command Spells of Bazett Fraga McRemitz are shaped like a double-sided Fragarach, which is also a sword, and Ryuunosuke Uryuu’s Command Spells remind him of intertwined snakes.
The seals are three absolute orders that can either restrain or reinforce the action of a Servant, to the point of making it perform an otherwise impossible action. If it is within the scope of the prana of the Master and the Servant, then it can be realized through the power of a Command Seal-backed order – even if it is something the Servant isn’t normally able to do. Truly impossible orders, such as those requiring real Magic, cannot be accomplished.
If a given order is meant to apply over a long period of time or if the order is vague, the strength of the order will grow weaker. Of course, the opposite works as well: an order that is very specific will be more effective and will hurt the Servant if he disobeys – although it is still possible to resist the order at the cost of ever-increasing pain. For example, an order such as “obey everything I say” is largely meaningless. The case of Rin, who used such an order on Archer the first night after he was summoned, is an exception, and it had some effect as she was a superior magus. Archer mentioned that he feels a weight if he disobeys her, and received a boost when he follows her orders. On the other side of the coin, early during the 4th Grail War, Kayneth Archibald El-Melloi issued a Seal-backed order to his Lancer to “help Berserker kill Saber”. This was something Lancer emphatically did not wish to do (desiring instead to duel her one-on-one and having saved Saber just moments prior), but because of both the power of the magus behind the order and the specific wording, Lancer experienced agony profound enough to cause visible convulsions when attempting to resist, and lasted mere seconds before being forced to give in.
If a Servant is fatally wounded, an order such as “heal right this instant” will result in the wound being temporarily sewed shut with a thread of prana, allowing the Servant to survive for a brief period; it does not, however, provide any actual medical treatment.
Even if the Master doesn’t consciously activate a Command Spell, it can automatically be used unless it is an impossible order.
Masters have three Command Spells that will vanish one by one as he/she uses them. A Master who uses up all three loses his or her right of authority over their Servant, but, practically speaking, this becomes a question of how close the relationship between Master and Servant is and how willing a Servant is to follow orders or assist anyway. If a contract is forcibly severed, the remaining seals lose their effectiveness and may disappear (although they do not in the case of Servant death). Loss of the Command Seals does not indicate forfeiture of position in the Grail War, and a Master and Servant may continue to participate long after the seals are used up, if they so choose.
When a Servant is killed, the color of the seals fade out and they look at best like a birthmark or a bruise, but they don’t disappear entirely. Even after the war has ended, the Command Spells remain, although a magus who loses their Servant to death also loses their status as a Master.
After the war has ended, it is possible to remain as a Master if the magus still has his/her Seals and his Servant.
Command Spells that disappear without being used are still stored within the Grail, so they can still be reused when making a new contract with another Servant who has lost his Master. But as there will normally be no more eligible magus, they will tend to come back to the previously selected Master. In the end, all the unused Command Spells go back to the care of the Supervisor. The Supervisor can freely redistribute them without any restriction, and transfer like he would a regular magus seal.