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It depends on which Monster Fest you mean and which version of the game you're referring to. The PC/Console versions are awarded a weapon for defeating the normal Monster Fest. On Medium and Hard you receive the Spirit Champion's Mageblade, and on Insane and Nightmare (only on PC) you receive Magina's Last Glaive. PC/Console users do not receive a Fairy on any difficulty, but Mobile users that complete this challenge on Hard or Insane do.