Reanimate recently popped enemies as servants to destroy the Bloons.
~ BTD6 description
Necromancer: Unpopped Army is the fourth upgrade of Path 3 for the Wizard Monkey in BTD6. It causes all bloons popped in the Wizard's range to become undead bloons that travel towards the entrance and deal damage to bloons they touch. This upgrade also allows the Wizard Monkey to pop Purple and Lead Bloons through the summoned undead bloons. Each undead bloon affects up to 2 bloons and deals 1 damage per hit. It costs $2,975 on Easy, $3,500 on Medium, $3,780 on Hard and $4,200 on Impoppable.