Army of Ruin

Necromancer Gauntlets is one of the starting weapons. You'll first need to get it to Level 5 in any stage to unlock the ability to evolve it. Then, during this stage, you need to get the Gauntlets (or play as Selena when you unlock her, because she starts with this weapon), level them up to max, and also get an Earth trinket and a Wind trinket. Once you've done that, open a chest (such as one dropped by a miniboss) to evolve the weapon and unlock this.