How do you make a potion of decaying in Minecraft?

To make a Potion of Decay in minecraft, you first need to create a Potion of Weakness. Start by brewing a Water Bottle with Blaze Powder in a Brewing Stand, then add a Fermented Spider Eye to the Water Bottle. This will yield a Potion of Weakness, which can be used to create a Potion of Decay by combining it with a specific ingredient, but note that as of the latest updates, a Potion of Decay does not exist in the game.