A Fermented Spider Eye is an item in the game Minecraft. It is created by combining a spider eye with a potion of any kind, which results in a unique item used for brewing. This item is essential for crafting certain potions, particularly those that have negative effects. In the game, the spider eye itself is obtained by defeating spiders or cave spiders. When fermented, the spider eye can be used to create potions like the Potion of Weakness. This potion is useful for various gameplay mechanics, including curing zombie villagers and enhancing certain strategies in combat.