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Fury is a status effect in Mario & Luigi: Bowser's Inside Story that can only be obtained by Bowser when he takes too much damage. When in this state, Bowser deals and receives more damage. The only way to cure this effect is to eat a Refreshing Herb, have a set amount of turns pass, or defeat the enemies on screen. While in the "Fury" status, Bowser's battle pose is different, he flashes red, and he shows more anger on his face. This status effect is basically a combination of POW-Up and DEF-Down.
Bowser also uses this status effect in Mario & Luigi: Paper Jam in the final battle with him and Paper Bowser. Choombas also go through a similar phase when fired up by Coalbits dealing more damage and with the addition of being a lot faster along with glowing red.