It's located in the Halls of the Dead, guarded by Bloodwitch the Wild. You will acquire the Horadric Cube as part of your questing through Act II.

Gear Crafting Recipes in the Horadric Cube For more information about the high-end crafting Horadric Cube recipes, please see the following guide:

These can sometimes rival end-game rares, and even exceed some Uniques or Set items if you get lucky on rolls. There are also four types of Armor and Weapons that can be crafted inside the Horadric Cube. You will find the Horadric Cube on the third and final level. You will obtain the Horadric Cube from the Halls of the Dead in the Dry Hills on your first trip through in Normal difficulty. It's even used to create the item required to open the famed Secret Cow Level! It's also involved in some quest lines, as it's needed to create items that open doors to new adventuring realms.

The Horadric Cube is an ancient item of Horadric design, used to transmute items such as gear, runes, gems, potions, and scrolls into more powerful versions of the original, add sockets to existing armor, and create specific magical items. Welcome to Wowhead's guide to Horadric Cube Recipes in Diablo II: Resurrected Patch 2.7 & Ladder Season 4.

Diablo 3 kanai cube ingredients