The Attitude-Reversing Potion (otherwise known as the Mood Potion) is a powerful potion that is able to change a person's attitude and their opposite side if consumed. It is safe with a small drop added to a drink, but should half the bottle be added then the outcome can disastrous; the potion can ware off on its own in time but it antidote can clear the affects much faster.
Role in the series
During the events of "Not in the Mood", Xavier had a bottle of the potion on hand and he gave it to Maximus and Pascal when they wanted to teach Rapunzel, Cassandra and Eugene a lesson. As well as to stop them from fighting. Xavier advised them to give the three a drop each, however, as Pascal was adding it to a jug of lemonade, he was startled by the three still arguing as they entered the kitchen, and accidentally drops the whole bottle into the drink. Once Rapunzel, Eugene and Cassandra drank from it the potion turned them into their opposite sides. Cassandra is overwhelmingly peppy, Eugene no longer feels confident about himself, and Rapunzel is all moody.
Since the affects didn't ware on their own, due to the amount that was consumed by the three, Xavier explained that it will be days before they return back to normal. Unless they take the antidote that is made from the oil from a rare three leafed echo plant, it grows among its two leaf cousin at the top of Mount Saison. After Pascal and Maximus came back with the ingredient they accidentally knock Xavier out old, so the two took it upon themselves to make the potion and to bring it to the castle; so they could cure Rapunzel, Eugene, and Cassandra. However, as they charge into the banquet to cure their friends, Pascal again drops most of the potion into Eugene's fog machine, which made the machine let out a huge burst of purple fog into the dining hall. The results may have returned Rapunzel, Eugene, and Cassandra back to normal, but everyone else becomes effected by the potion. Arianna becomes ill mannered, Nigel cannot stop laughing, Pete and Stan began to fight each other, the Captain becomes childish, and Frederic is overwhelmingly tyrannical.
Not long after Rapunzel, Eugene, and Cassandra forgave each other and realized that Pascal and Maximus were just trying to teach them a lesson, they took it upon themselves to fix the damage caused by the potion. Once everyone was back to normal and not long after bidding their guest, the Griffin of Pittsford farewell as he sails home, Rapunzel threw the bottle and what is left of the potion into the sea, much to Maximus and Pascal's relief, so it wouldn't cause anymore harm. However, instead of sinking to the bottoms of the sea, it flouted above the waves as the currents brought it to Old Corona, where it is discovered by Varian.
As there was a small drop left in the bottle, Varian was able to use his alchemy to replicate, once he had discovered its power, and modify it into a truth serum. Which he used, after hearing a rumor about the Magical Golden Flower still existing, as he mixed it with a batch of cookies and handed them to everyone in the events of "The Alchemist Returns". While there were sights of the potion being used again, from the way people were acting, it wasn't truly revealed until Rapunzel discovered its empty flocking in Varian's possession.