command spell
A symbol from the Fate series depicting a Master's control over a particular Servant, often seen as a red mark on the Master's hand of various shape. Jeanne D'arc bears a large one upon her back as a sign of her Ruler class privilege to command all servants in the Grail War.
The Command Spell can be used by the Master to force the Servant to perform any actions they wanted, even if it's against their will, such as forcing them to commit suicide.