Mouthpiece - Borderlands 3 Farming Guides
Location: Ascension Bluff on Pandora
Part of: Cult Following Story Quest
Drops: Nemesis (Dahl Pistol) and Mind-killer (Maliwan Shotgun)
Mouthpiece is the first real boss that you face in Borderlands 3; he's found in Ascension Bluff on Pandora in the Holy Broadcast center (shown on the map above). Once you defeat a boss in Borderlands 3 all you need to do is exit/enter the game then return to the bosses arena to fight them a second time.
As far as loot goes, Nemesis (Dahl Pistol) and Mind-killer (Maliwan Shotgun) are both on his dedicated drops loot list. You can find the Nemesis pistol easily enough elsewhere in the game but the Mind-killer is a drop unique to Mouthpiece and is difficult to find off other enemies.