When you're in vampirism's batform, since the batform does not allow you to place blocks, when you place the last skull on a wither structure it spawns the wither, but keeps all of the needed blocks. This allows you to spawn multiple withers without using any materials.
When you're in vampirism's batform, since the batform does not allow you to place blocks, when you place the last skull on a wither structure it spawns the wither, but keeps all of the needed blocks. This allows you to spawn multiple withers without using any materials.