Check this Monster Hunter World: Iceborne guide on how to unlock Bandit Mantle! Know the effects of Bandit Mantle and which quests you need to complete with this guide!
Bandit Mantle - Effects & How To Unlock
Bandit Mantle Equipment Info
Bandit Mantle Effects
Causes monsters to drop rare trade-in items when you attack them.
Bandit Mantle Usage Info
Bandit Mantle - How To Unlock
Clear ★5 Optional Quest: Redefining the "Power Couple", then talk to the smithy.
Bandit Mantle - How To Use
Earn Money by Quickly Acquiring and Selling Trade-in Items
Attacking while the Bandit Mantle is equipped causes the monster to sometimes drop trade-in items. These trade-in items can then be sold for 5000z, so if one intends to quickly farm currency, equipping the Bandit Mantle is a must.