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The Avenger bugs are a series of bugs involving the weapon Avenger in Final Fantasy IV. Several bugs include the following.
Weapon properties and stats don't change when equipping the Avenger in combat (affects all SNES versions)
Equipping the Avenger while in combat doesn't change the previous weapon's stats to the Avenger. The result is effectively like casting Berserk on the character. The previous weapon's attack power and elemental effects are preserved. This bug is only useful to avoid the Crystal Armor's Berserk protection.
Kain 'casts magic' while equipped with the Avenger in Zeromus battle (affects FF2US [both 1.0 and 1.1] and FF4)
When Kain equips the Avenger, there is some strange interaction with the Avenger's reapplying Berserk and the hidden "Black Hole" spell that comes with Zeromus shaking. It requires special timing to pull off. If someone precedes Kain by casting a spell (or using an item), and Zeromus does not shake before Kain's turn, Kain may attempt to cast a "dummy" spell (although, in the case of the item, Kain repeats using the same item). It seems the Avenger makes Kain repeat the last thing that was executed. This is fixed in the EasyType version.
Character wielding Avenger doesn't initially automatically attack (affects all SNES versions).
When equipped with the Avenger, the character will not start attacking until its "logical" turn has occurred. To work around this bug the player can place the character in the middle slot so the character will act normal. This is fixed in the Advance version.
If the player has Cecil equipped with the Avenger in the original version of Final Fantasy IV's final battle, the battle becomes unwinnable since Cecil's Auto-Berserk prevents him from using the crystal.