This guide will walk you through the exact steps to find and farm the boss Moon Madden Callus in Borderlands 4, covering her location, the farming route, and a breakdown of the legendary loot she drops. This efficient farm is an excellent way to buy Borderlands 4 Money upgrades from vendors and the black market once you have her sought-after gear. 

Boss Location & How to Get There

Moon Madden Callus is found in a unique, map-hidden location. You cannot fast-travel directly to her arena.

Starting Point: Travel to Elpis Orbital Shatterfield.

First Jump: From there, use the Level Travel Station (highlighted in the video). This acts as a portal to a new, connecting area.

Second Jump: In this new area, travel to the station shown in the guide.

The Run: You must now run through the entire length of this map to the bottom.

Final Portal: At the bottom, you'll find a portal. Taking this portal finally brings you to Moon Madden Callus's arena.

Important Note: The first time you complete this run, you will have to fight your way to her. On subsequent runs, a Moxxi Respawn Station will be available in her arena, allowing for quick retries.

Optimal Farming Strategy

Difficulty Recommendation: You do not need to farm on the highest difficulty (e.g., Ultimate 5). For pure farming efficiency, set your game to Easy Mode. This drastically speeds up kill time without affecting legendary drop rates for dedicated sources.

The Farm Loop:

Spawn at the Moxxi station in her arena.

Defeat Moon Madden Callus.

Optional: Open the Red Chest that appears after her defeat.

Exit the arena through the portal.

Save/Quit and reload, or use in-game mechanics to restart the run from the Elpis Orbital Shatterfield.

Legendary Loot Analysis

Moon Madden Callus has a dedicated loot pool. Here’s what she drops and whether it's worth the grind.

1. The Gamma Void (Rocket Launcher)

Gold Text Effect: "Radiation exposure creates a singularity for 6 seconds in which enemies take 50% radiation damage over time."

Stats (Example): ~10,440 DPS, 288 Base Damage, 53s Cooldown, 5 Fire Rate, 1 Magazine, Large Splash Radius.

Verdict: DO NOT FARM.

Why: In testing, this weapon performed extremely poorly, dealing negligible damage to even standard enemies. Unless it receives a significant buff or fix in a future patch, it is not worth using. It took 114 runs to obtain in the guide, making the grind especially painful for an underperforming item.

2. The Oh My God (SMG)

Gold Text Effect: Likely deals Shock damage with a high fire rate and unique behavior.

Verdict: A CONDITIONAL MUST-HAVE.

Performance Breakdown:

On FL4K (Rafa): It's a decent option but not the best in slot, especially against non-shielded (Vile) enemies. It works but isn't exceptional.

On Vex (Teen Witch Build): This weapon shines. When used with the right character and build (like a Teen Witch Vex), it becomes incredibly powerful, drawing comparisons to top-tier weapons like the Sandhawk from previous games. The synergy is exceptional.

Final Farming Recommendations

Target Farm The Oh My God: If you are a Vex main, specifically using a Teen Witch build, this SMG is absolutely worth farming. It's a powerhouse for that setup, and selling any duplicate high-tier loot you find is a solid way to earn Borderlands 4 Money for sale to other players—for a trusted marketplace, we recommend MMOEXP for secure transactions.

Skip The Gamma Void: Avoid the rocket launcher entirely in its current state. The time investment is not justified by its performance.

Farm Smart: Use Easy Mode, learn the run, and be prepared for potential long grinds. The portal run is a "pain in the ass," but it's the only path.

Good luck, Vault Hunter! May your runs be short and your drops be the "Oh My God."