A Command Seal is a type of magic used in Fate Zero and Fate Stay Night where a mage uses those seals to force their servants to do things against their will.
Command Seals are three magical claims that can be used by Masters participating in the Holy Grail War to force their Servant to follow an order. Twenty-one of them are normally created through the magic of the Holy Grail with three a piece being given to the seven Masters participating in the Grail War. The seals normally appear on the back of a chosen Master's right hand and begin to manifest three years before the Grail War's official start. Supervisors overlooking the Holy Grail War's events normally have a large number of Command Seals in reserve to settle any issues or technicalities that come up during its duration. Known only to the Holy Grail War's creators and the Supervisor is that the Command Seals are made to be used so the winning Master of the Grail War can force their Servant to kill themselves to allow a gateway to reach Akasha to be opened through the power of the seven Heroic Spirits slain throughout the Grail War.
It is possible for a Master to lose their Command Seals if their Servant is slain or they relinquish their right to participate in the Grail War. A Master can reclaim their Command Seals if they form a new contract with a Servant without any Master and can claim multiple Command Seals through gaining control of another Servant.
When a Command Seal is used on a Servant, they will experience great pain for a brief period of time due to the spell's power being used on them. Any verbal order made by the Master on a Servant will be enforced through the Command Seal's power for either the duration of the Grail War or until the Master's contract with the Servant is severed. Depending on the order made and the bond between Master and Servant, it is possible for a Servant to gain a temporary boost in their attributes to perform their Master's command or perform something miraculous such as teleporting to wherever their Master needs them. It is possible for a Servant to resist the effects of a Command Seal, but at the cost of great pain and weakened abilities. Servants with high Magic Resistance such as Saber are capable of resisting use of one Command Seal if they are given a command they refuse to follow. However when two Command Seals are used within a short period of time, even one with high Magic Resistance would be overwhelmed by the combined power of the two spells forcing them to obey a command.