Potion Door Trap is used to kill the mobs. Whenever a mob passes through this gate, the splash potion of harming will shower on him that provides some serious damage.
This guide will let you know how to make a Potion Door Trap in the Minecraft game.
First, you will need to place iron blocks on the ground in the same way as shown in the image below. And add an iron door on the front side.
Place a stone button on the left side of the door and a redstone torch right at the back of this buton. Add a block of iron at the top and add one more redstone torch.
Place a dispenser at the top and fill it with a splash potion of harming.