Berserker

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 during battle at least once in their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their 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.

It is possible to specify in the summoning ritual that the Servant be placed under the Berserker class for those familiar with the workings of the Grail System. By adding in a specific line, Zouken Matou and the Einzbern family, as members of the Three Founding Families, were able to choose the class during the Fourth and Fifth Wars. This class comes at a price, as the Berserker class is the most difficult to control and a Master must expend a large amount of additional magical energy merely to maintain the Berserker, and the affected Heroic Spirit may lose access to certain abilities or even Noble Phantasms depending on how much sanity it taken away for power.

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. Kariya Matou is drained of his energy at a high rate and eventually it causes his death, but he could have handled another class without difficulty. They have generally brought about their own master's ruin due to their inability to control them, so the Einzbern family developed Ilya's Magic Circuits specifically so that she could handle the strain with ease in the Fifth War.

Known Berserkers
Servants marked with ‡ are also able to be summoned by the Protagonist in the Grand Orders.

Nonstandard Berserkers
Servants possessing the class of Berserker for special reasons unrelated to a proper summoning.