A fermented spider eye is an item in the game Minecraft. It is created by placing a spider eye in a brewing stand along with a specific ingredient, such as a potion or another item. This process transforms the spider eye into a unique potion ingredient that can be used to create various potions with special effects. In Minecraft, spider eyes are obtained by defeating spiders or cave spiders. When fermented, they can enhance potions, making them more potent or giving them new properties. Players often use fermented spider eyes to craft potions like the Potion of Weakness, which can be useful in various gameplay scenarios.