The Bull King's Iron Horn is a mythic boss drop found in Bishui Cave, a secret area in Chapter 5. You'll need this material to craft the Bull King's Armor set, which includes a mask, armor, bracers, and greaves. The mask grants moderate Focus when taking damage, and when staggered, it grants even more Focus. The bracers and greaves grant bonuses to cold and fire resistance, and the Bull King's Shan Wen Armor grants good defense and more effects.
All locations of The Bull King's Iron Horn Black Myth Wukong
If you haven't unlocked this region yet and want to know how to achieve it, this guide is for you.
You can obtain the Iron Horn from Bull King by opening a chest in Cave Depths in Secret: Bishui Cave in Chapter 5.
During the same chapter, you can defeat the Bishui Golden-Eyed Beast boss to get 3 of this material as a drop. It also drops Mind Cores, Fine Gold Thread, Celestial Ribbon, Bishui Beast Bit, and Daoist's Basket of Fire and Water Curio.