How to Brew a Potion of Decay in Rewithered

Gather the Base Ingredients

Start by brewing a standard Awkward Potion at a Brewing Stand using Nether Wart and a Water Bottle. You will also need Wither Roses, which are the key modifier that turns an awkward potion into a Potion of Decay. Wither Roses spawn naturally in the Nether near wither-related structures and can be obtained by letting a Wither kill a mob, or by finding them in the wild. Keep a few on hand since you may want to brew multiple variants.

Brew the Potion

Place your Awkward Potion into the Brewing Stand and add a Wither Rose as the ingredient. This converts the awkward potion into a Potion of Decay, which applies the Wither status effect when consumed. From here you can extend it with Redstone for a long variant, or amplify it with Glowstone Dust for a strong variant. Note that strong and long variants are mutually exclusive, so choose based on whether you want longer duration or higher potency.

Convert to Splash or Lingering

Add Gunpowder to make a Splash Potion of Decay for throwing, or add Dragon's Breath to make a Lingering Potion of Decay. Lingering versions are especially useful because they can be crafted into Arrows of Decay, letting you apply the Wither effect at range.