Berserker (バーサーカー, Bāsākā?), Mad Warrior, is one of the seven normal 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.
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 Master must expend a large amount of additional prana merely to maintain the Berseker, 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. As a result, in every Holy Grail War before the fifth, the prana consumption and insanity of the Berserker class has made them incredibly difficult to maintain and control. Kariya Matou is in constant pain and suffering during the Fourth War, and eventually played a large part in his death. 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.
- Details are unknown for the Berserker class Servants summoned during the First and Second Wars, and no Servant was summoned under the class during the Third War. Avenger was summoned in its place.
- The Berserker Class Servant summoned during the Fourth War in Fate/Zero is Lancelot summoned by Kariya Matou. While the class was forced upon him by Zouken Matou, Lancelot himself kept the Mad Enhancement ability activated to run away from his own sorrow.
- The Berserker Class Servant summoned during the Fifth War in Fate/stay night is Heracles summoned by Illyasviel von Einzbern. He was placed under the class forcibly in order to have a loyal Servant incapable of betraying them due to the recent treachery of Kiritsugu Emiya. Ilya triggered his Mad Enhancement shortly after his summoning and always kept it active in spite of the pain it was causing her. Cú Chulainn mentions to Shirou that he also could have been summoned as a Berserker class Servant because of his savagery in battle.
- The Berserker Class Servants summoned during the Holy Grail War of the Moon Cell in Fate/Extra include Lu Bu summoned by Rani VIII, and Arcueid summoned by Gatoh Monji. Lu Bu has among the highest Mad Enhancement possible, rendering him almost mechanical. Arcueid is severely weakened due to her Master's misunderstanding, and it is said her original class would have been "Funny Vamp." Li Shuwen, summoned as Assassin, temporarily falls under the Berserker class after he is defeated in battle.
- The Berserker Class Servants summoned during the Holy Grail War of Fate/Apocrypha are Frankenstein and Spartacus. Sakata Kintoki was planned for the cancelled Online Game project. Jack the Ripper, summoned as Assassin, also has the qualifications to be summoned as Berserker.
- The Berserker Class Servant summoned during the non-canon Holy Grail War of Fate/strange fake is Jack the Ripper summoned by Flat Eskardos. It is a version of Jack the Ripper different than the one of Fate/Apocrypha, and it seems to have been turned sane due to Mad Enhancement.