Leader Willem is the boss at the end of the Electi side-mission chain in Carcadia Burn. You reach his fight by completing five missions in order, the last of which is The Mole’s Gambit, and the encounter sits near The Excrucible, east of Grindstone of the Worthy. The route, loot and combat details below come from published guides, not from firsthand play, and they are dated 2025, so confirm quest names and drop lists in your own game.
Spoiler note: the quest steps below name the mission that leads directly into the boss fight.
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Where Willem is and how to unlock the fight
Nerd’s Chalk and PC Gamer both place the fight in Carcadia Burn, around The Excrucible, east of Grindstone of the Worthy and near the route toward Terminus Range. Finding the arena is the easy part; the fight only becomes available once the mission chain is finished.
The five-mission chain
Nerd’s Chalk lays the sequence out in this order. PC Gamer adds that the chain starts with Levaine in Carcadia.
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- Enter the Electi Part 1
- Enter the Electi Part 2
- The Council Divided
- A Traitor Within
- The Mole’s Gambit, which ends with the Willem fight
Why shorter summaries look different
Mobalytics describes the route more briefly: progress the Carcadia Burn story until Enter the Electi, complete the side quest, and the chain eventually leads to Willem. That wording compresses several intermediate missions into one step, so it can read as if the first Electi quest opens a boss marker by itself. It does not. The five-step list above is the clearest route the guides provide.
Repeat encounters
PC Gamer reports that killing Willem at the end of the chain unlocks Moxxi’s Big Encore, which is what lets you fight him again. That repeat-fight option is the basis for every farming approach discussed below.
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How the fight works, according to guides
Mobalytics describes Willem’s health in two layers: a blue shield bar and a yellow armor bar. Its recommendations, which are guide advice rather than tested results, are:
- Shield: use Shock or Radiation first.
- Armor: switch to Corrosive or Cryo once the shield is down.
- Positioning: use range and arena obstacles. Mobalytics notes that much of his attack set can be dodged or outranged rather than tanked.
- Phase two: expect an area attack centered on Willem, so keep your distance when it begins.
MMOGAH also recommends Shock for the shield and Corrosive for armor, which lines up with Mobalytics, though that guide’s publication date is unclear.
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- FIGHT SOLO OR CO-OP: Wreaking havoc across Kairos is awesome alone and even better with friends in 2-player splitscreen or up to 4-player online co-op.** Borderlands 4 is designed for co-op from the ground up; whether you're hunting for loot, tackling missions, or wandering freely, level scaling and individual difficulty keeps the party together and having fun.
- LESS BORDERS, MORE LANDS: Freely explore a vast and dangerous world rife with warring factions. Hop on your hover bike and ride through lush fields, towering peaks, and deadly deserts full of fearsome enemies, dynamic events, and engaging quests with unforgettable characters. Unite the people of Kairos and ignite a revolution, tackling this adventure however you see fit in a seamless Borderlands experience.
Items guides associate with Willem
The table below reflects what each guide says, not a verified drop table. Stats are quoted as the guide reports them and have not been independently checked.
| Item | Type | Described in guides as | Source |
|---|---|---|---|
| Rainbow Vomit | Shotgun | Fires droplets of three elemental types; the combination varies by drop. Its Colour Spray perk fires the three elements at once. | Mobalytics (updated December 17, 2025); PC Gamer (September 17, 2025) |
| Zipper | Pistol | +75% critical damage and +75% damage on back hits | Mobalytics (updated December 17, 2025) |
| Firepot | Grenade ordnance | Spawns homing Torgue stickies over time | Mobalytics (updated December 17, 2025) |
Rainbow Vomit
Rainbow Vomit is the item most readers are chasing. PC Gamer reports it can also appear as a world drop outside the Willem fight, so Willem is not the only place to find it. Its mixed-element behavior means two copies can feel quite different, which is why a specific roll is harder to target than the item name alone suggests.
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- FIGHT SOLO OR CO-OP: Wreaking havoc across Kairos is awesome alone and even better with friends in 2-player splitscreen or up to 4-player online co-op.* Borderlands 4 is designed for co-op from the ground up; whether you're hunting for loot, tackling missions, or wandering freely, level scaling and individual difficulty keeps the party together and having fun.
- LESS BORDERS, MORE LANDS: Freely explore a vast and dangerous world rife with warring factions. Hop on your hover bike and ride through lush fields, towering peaks, and deadly deserts full of fearsome enemies, dynamic events, and engaging quests with unforgettable characters. Unite the people of Kairos and ignite a revolution, tackling this adventure however you see fit in a seamless Borderlands experience.
Zipper and Firepot
Both items are listed only by Mobalytics among the guides reviewed, and MMOGAH also lists them without consistent stat claims. MMOGAH’s evaluations, including any statement that an item is “not worth farming,” are opinion rather than settled fact.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is farming Willem worth your time?
Willem is worth another run if you specifically want one of the listed items, want another roll on a piece of gear you already have, or simply enjoy the encounter. He is less compelling if your only goal is Rainbow Vomit, because PC Gamer reports the shotgun also drops in the world, and ordinary play may get it sooner.
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The guides reviewed do not cite official drop rates, a numeric chance per kill, or drop data for the current patch. Do not expect a fixed number of runs. MMOGAH’s estimate of 30 to 40 kills is an anecdotal, editorial figure, not a reliable expectation.
Three questions will settle the decision for most players:
- Have you unlocked Willem? If not, the five-mission chain comes first.
- Do you need that exact item, or any Legendary? If any Legendary will do, a broader approach works.
- Do you prefer a targeted repeat fight or varied play? Willem is repetitive by design; world drops and other activities are not.
2K’s endgame announcement describes two general systems. Weekly Wildcards offer a guaranteed Legendary reward, and the Weekly Big Encore selects a boss for a larger version with increased drop rates after an Eridium unlock. Neither system is confirmed to include Willem this week, and weekly rotations change, so check the in-game map before planning a run around them.
Lore
Nerd’s Chalk describes Willem as an important side-mission character involved in efforts to restore and control Carcadia’s urban hub. The guides reviewed do not quote his dialogue, a developer statement about his motives, or a detailed backstory, so treat the story role as the guide’s summary rather than a full account of his place in the plot.
Dates and what to check in your game
- Nerd’s Chalk was updated September 22, 2025.
- PC Gamer’s Rainbow Vomit guide was published September 17, 2025.
- Mobalytics’ boss guide was updated December 17, 2025.
- MMOGAH’s guide has no clear publication date.
Borderlands 4 has received patches since these guides were written. Quest names, boss behavior and drop tables may have changed, so check the mission log and the boss’s on-screen bars before following the steps above.
Quick Recap
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