The Fermented Spider Eye is a common ingredient you will need to craft when brewing a Potion of Weakness, Harming, Slowness, or Invisibility in Minecraft. Naturally, a required resource for this item will be a Spider Eye, mob loot with a roughly 30% drop rate from Spiders and Cave Spiders. Be careful when dealing with these hostile critters, as they are quite nimble and can overwhelm a sole adventurer in numbers if precautions are not taken. There is a 25% chance that Spider Monster Spawners will appear in Dungeons or Cobweb Rooms of Woodland Mansions.
Crafting a Fermented Spider Eye in Minecraft
Once you have procured a Spider Eye in Minecraft, the next two ingredients you will need to craft a Fermented Spider Eye are a Brown Mushroom and a stack of Sugar. Sugar comes from Sugar Cane. Brown Mushrooms generally spawn in dank or poorly lit areas, like swamps or caves.
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According to the Minecraft Wiki, Sugar Cane can generate in any biome where water is present. However, the chance that this crop will appear is increased for Swamp biomes and exponentially raised for Desert biomes. In other words, you will have a much easier time finding Sugar Cane in one of these two biomes than in forests or plains across the Overworld. Wandering Traders can sometimes sell Sugar, but typically such a purchase is not a good investment in your Emeralds. The best way to farm Sugar Cane would be to live near a Desert where the resource is present or gather some to plant near a body of water by your home. Be sure to never harvest all the Sugar Cane to ensure it still grows back.
The last ingredient you will need to craft a Fermented Spider Eye in Minecraft is the Brown Mushroom. Mushrooms often appear in Swamps, Old-growth Taiga biomes, and Mushroom Fields. You can also find Brown Mushrooms while exploring the Nether. Remember that only the brown variant of fungi can be used for the Fermented Spider Eye. The red-and-white variation is different and cannot be used in the recipe.
Minecraft is available on PC, PlayStation 4, PlayStation 5, Xbox One, Xbox One X|S, Xbox Series X|S, Nintendo Switch, Android, and iOS.
Source: Minecraft Wiki