FERMENTED SPIDER EYE

Everything else in this group of twelve falls out of a mob when it dies. This one does not: A crafting grid makes one from 1 × spider eye, 1 × brown mushroom and 1 × sugar. It is a brewing ingredient before it is ever a trade good, which changes the question this page answers. It is not where does it come from, it is why would you sell a potion component instead of using it.

The Wandering Trader will pay 3 emeralds for one, one of 6 items on his buying list, and he is the only villager interested. No profession buys or sells it, which keeps the trade a curiosity rather than a strategy for most brewers.

The honest case for selling it is overflow. A brewing setup that keeps a steady supply of spider eyes, mushrooms and sugar going will occasionally end up with more fermented spider eyes than the current batch of potions needs, and a passing trader is one way to turn that surplus into emeralds instead of letting it sit in a chest.

3 emeralds a piece, fixed rather than rolled, for up to 2 trades, which puts 6 emeralds on the table for a trader's full interest in it and closes the offer there.

Against the rest of his buying list it sits in the middle of the pack. Nothing else he buys pays more per item, so it is a fair rate rather than a standout one. No job villager offers anything for it at all, in either direction.

The Wandering Trader has no village and no tier ladder, so the usual moves for working a profession, breaking a workstation to force a reroll or curing a villager for a discount, do not exist here. He spawns near a player with two llamas, stays a short while, and leaves on his own schedule.

His buying lines are set the instant he spawns, 2 drawn from 6 possible entries, so a given trader either wants fermented spider eyes that day or he never will. Meeting more traders is the only way to raise the odds of finding one who does.

When one does want it, the whole exchange runs out after 2 trades, for a maximum of 6 emeralds. Beyond that the slot is closed for good, which makes this a trade for clearing brewing overflow rather than one worth planning a route around.

A spider eye, a brown mushroom and sugar make one fermented spider eye, and its only job is corruption. Add it to a potion and you get that potion's opposite: healing becomes harming, swiftness becomes slowness, night vision becomes invisibility.

That makes it the ingredient with no potion of its own. Everything else in a brewing stand adds an effect; this one takes the effect you already brewed and flips it, which is why the recipes people memorise are really two recipes with this in the middle.

Weakness is the exception worth knowing. Fermented spider eye in plain water makes weakness directly, no base potion needed, and that is the potion a zombie villager has to be splashed with before it can be cured.