Wither Essence

The Wither Essence is an odd material gained only from extracting a Nether Star in the Extractor. This process destroys the Nether Star. If you can two of these into a conventional emerald, you acquire another Nether Star. Crafting this and a Nether Wart gives the ever-enduring plant called the Wither Wart. More info about that on its page. It can be used to create the Potion of Withering, which embodies the power of the Wither.