When Magic Booster is used in the 2013 PC re-release, the player's inventory gains 100 Berserk spells, among other basic spells.
Berserk can be drawn from a small variety of enemies. It can be most easily refined from Steel Pipes using the Guardian Force Diablos' ability ST Mag-RF. GIM47Ns and Wendigos (found in forests near Timber) are a good source of Steel Pipes, as well as Elastoid cards. Though Magic Stones can be refined into Berserk spells also, Magic Stones are often better used on other spells.
Berserk when cast will inflict the Berserk status effect on a target that is not immune to it. This renders the target unable to control their actions until the status effect is removed, causing them to attack random foes, with a raised physical attack damage of 50%.
Berserk can be cast using the Magic command, Selphie's Slot Limit Break, or Rinoa's Angel Wing if she has it in her inventory. Casting Berserk in battle increases compatibility with Doomtrain by 1 and with Siren by 0.6, but lowers compatibility with Cerberus and Carbuncle by 0.2.
Berserk can be junctioned to ST-Atk-J to inflict Berserk on enemies with a physical attack. Though Berserk can be useful in some cases against enemies with very weak physical attacks who normally prefer using magic, it is often better to simply junction Silence or Sleep for a more versatile effect.
Berserk is better junctioned to ST-Def-J to resist against Berserk, as 100 Berserks can lead to immunity to it. This is especially important against enemies that can inflict Berserk, such as Malboros, as it can be a potentially devastating status, especially since Berserk, unlike Confuse or Sleep, is permanent.
In some instances, the player may wish to cast Berserk on their own party to increase physical damage if they only intend to attack. In these cases, the ability Mad Rush is a better choice, as it casts the effect on all party members at once in addition to Haste.