Borderlands 3
The Tenderizer

The Cartels Event was made permanent on the 24th of July 2021.
It can now always be activated in the Main Menu.

Any of the six underbosses can spawn at 4 set locations in the Villa Ultraviolet.
2 spawn during the fight with Joey Ultraviolet.

Additionally, an underboss can spawn after you killed a neon-glowing Cartel Operative anywhere in the galaxy.

Underbosses:
‍FISH SLAP!!!, Franco Firewall, Josie Byte, Roaster, The Tenderizer, Tyrone Smallums