Foul Potion has three uses: deal 12 damage to all players and enemies in combat, obtain 100 Gold from a real Merchant, or start the hidden fight at The Merchant???. The lookalike Merchant is an event encounter with knockoff relics. Throwing the potion there commits you to combat rather than the ordinary Gold exchange.
Foul Potion and Merchant rooms
Potion Courier offers three Foul Potions or one random Uncommon Potion. The Foul Potions occupy potion slots, so consider what you must discard to carry them. Three bottles can be valuable with a shop ahead, but replacing an emergency defensive potion before a forced elite can make the route worse before you reach that shop.
At a real shop, close the inventory before throwing. The potion gives 100 Gold; it is not a general system for selling other potions. At the fake Merchant, the same action starts the fight. Check the Merchant’s displayed name and the stock instead of assuming the potion will always be a harmless payment.
The fake event appears from Act 2 onward in the stable event data, with a requirement of at least 100 Gold or a Foul Potion. It is listed for solo runs. Planning a multiplayer route around finding this event is therefore a poor reason to carry a bottle.
Purchases before the hidden fight
Winning the fight gives 300 Gold, the remaining unsold knockoff relics and The Merchant’s Rug???. The rug itself does nothing. Buying the stock before a fight you intend to win spends Gold on rewards you would otherwise receive afterward.
That is not an instruction to fight with an unprepared deck. The Merchant??? has 165 base HP and can gain Strength. His attacks include Swipe and a repeated coin attack, while Throw Relic applies Frail. A deck relying on several modest Block cards should account for that debuff. Temporary Strength reduction can help against repeated hits, but a long fight lets his buffs matter again.
Read knockoff descriptions individually if you choose to shop. Mango??? gives three Max HP; Anchor??? starts fights with four Block. Snecko Eye??? starts you Confused without the full original relic’s benefits. A familiar icon and lower price do not imply a proportionally discounted version of every benefit.
Using it as combat damage
The 12 damage also hits you and every partner. Block absorbs that damage, and Strength does not increase the potion’s damage. An apparent lethal against the last enemy still needs a survival check on your side. In co-op, confirm everyone’s health and Block before throwing.
The alternative is the known 100-Gold shop use. Spending the bottle to avoid a dangerous additional enemy turn can be worthwhile; spending it on harmless cleanup gives up that Gold and occupies the same decision space as your other potion timing choices.