Fourier's Malady is a 
Tediore Shotgun
Its special effect is Discontinouous - On fire, has a Chance to throw a copy of your Gun.
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How to Unlock The Legendary Fourier's Malady Tediore Shotgun
Weapons
Legendary Loot
Overview
How To Unlock
Unlock Location:
- Progress through the (FL4k and the Last Resort) DLC until reaching the Boss
Lieutenant Bume
Farm Method:
- Defeat Lieutenant Bume to get a chance at the Fourier's Malady.
- After use the Encore Machine nearby to reset the fight.
- Repeatedly defeat Lieutenant Bume until the drops Fourier's Malady.