Berserker (バーサーカー), Mad Warrior and Heroic Spirit of Berserker Rage, is one of the Four Cavalry classes among the seven standard Servant classes summoned for the Holy Grail War. Servants placed within this class are always Heroic Spirits who have gone berserk during battle at least once in their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their consciousness and sanity for a large power boost. This class is usually for magi who have summoned Heroic Spirits with insufficient abilities and attributes, allowing them to strengthen their Servants to make up for their lower attributes and compete against stronger opponents. While it is ideal for Servants to reach the level of the Heroic Spirit, the class allows for their attributes to surpass even the original. If used on an already strong Heroic Spirit, such as Heracles, they will become empowered to truly extraordinary levels. The abuse of this to create powerful Servants has made the class referred to as the strongest of all seven classes.

Masters are unable to utilize coordination with their Servant unless gifted with superb control over them. The lack of complex rational thinking keeps them from being able to follow more than simple directions, and it may even require Command Spells to keep them from acting on their own. They generally rely on little other than brute strength and overwhelming, uncontrolled strikes to physically beat down their opponents. As a result, in every Holy Grail War before the fifth, the magical energy consumption and insanity of the Berserker class has made them incredibly difficult to maintain and control.