The Foul Potion Has 3 Uses

The most context-sensitive item in the game — combat damage, merchant gold, and Fake Merchant boss trigger.

The Foul Potion is the most context-sensitive item in the game.

In combat, it deals 12 damage to all enemies.

At a real merchant, throw it at the shopkeeper to receive 100 gold (with a slime impact VFX).

At the Fake Merchant, throwing it reveals the impostor and triggers a boss fight against the Fake Merchant Monster (165-175 HP). Beat it to earn The Merchant's Rug??? relic and all unpurchased fake relics.