All side quests that unlock hidden bosses in Borderlands 4

Borderlands 4 features a good number of bosses with dedicated drops that you can only unlock, if you have completed certain side quest or questline, in some cases.

To make sure you are not missing out on any boss and are aware of all the side quests that unlock these bosses, I prepared this guide. I hope its helpful.

Side quests that unlock bosses

In Borderlands 4, there are five bosses that you can unlock by completing certain side quests and questlines. These are:

1. Leader Willem
2. Corrupt Ted, Experiment 42, & Experiment 1709
3. Bio-Thresher Omega
4. Genone
5. Sludgemaw

How to unlock Leader Willem

To unlock the Leader Willem boss in Borderlands 4, you must complete the Electi questline that you can start at the above marked location.

The quests in the questline include:

1. Enter the Electi (Part 1)
2. Enter the Electi (Part 2)
3. The Council Divided
4. A Traitor Within
5. The Mole’s Gambit

Talk to Leader Kassandra at the Ruined Sumplands Safe House and you will be able to start the Enter the Electi (Part 1). From there, you can just follow the questline till completion to unlock the boss.

How to unlock Corrupt Ted, Experiment 42, & Experiment 1709

You can unlock Ted and the Experiments boss by completing the “Below the Surface” side quest found in the Terminus District region of Dominion.

Head to the above marked location and talk to Ted to start the quest.

How to unlock Bio-Thresher Omega

To unlock the worst boss, the Bio-Thresher Omega, you must complete two quests:

1. Gassy Issue
2. Slimey Issue

To start the questline, head to Lakeside Properties area in the Fades District and talk to Beth.

How to unlock Genone

To unlock Genone, you must complete the “Fault Hunting” side quest found in the Ruined Sumplands region of the Carcadia Burn. At the above marked location.

Head there and talk to Leopold to start the quest.

How to unlock Sludge Maw

To unlock Sludgemaw, you must complete two quests:

1. Null and Void
2. All Charged Up

You can start Null and Void quest by talking to Rush at the above marked location.

That sums up all the hidden bosses in Borderlands 4 that you must unlock by completing side quests and questlines. If I missed any boss, please let me know in the comments.