Minecraft wiki entry for Skyblock, associated text: To make a witch give Weakness to your zombie villager, you will need to get near the witch with it and have less than 8 health points. There will be then a 25% chance that the witch will throw a splash potion of Weakness. To make your own potion, you need to get a brewing stand, a fermented spider eye and some gunpowder. Note that you will need a brown mushroom to do that, which requires trading with a wandering trader, which itself requires an emerald. In turn, this method is highly impractical for the first villager conversion.