The Green Gala was a Gas

When Cüper heard about the ritual that takes place at the Green Gala, he immediately started hatching a plan to have some fun with it.
He started asking around for information was able to learn a lot about the ritual and goblets, but also that pulling anything off when the goblet is spilled would be tricky. He didn't have any practice with remixing potions meant to be drunk to work when delivered as gas.
The first attempt didn't go too well, as it vaporized right away and made Cüper turn into a gaseous form right there at the alchemy station. Though he did enjoy the sensation, and was sure Untermauler would as well, he would need some extra help to get the formula correct. He was able convince his alchemy teacher to give him some general assistance about re-formulating his potion and managed to perfect his Gaseous Potion of Gaseous Form.
On the night of the Gala, he was able to slip some of his potion into few of the ritual goblets, but in his excitement in the moment, he failed to consider, or even remember, which goblets he need to spike to get Untermauler. When the ceremony came, instead of Untermauler getting gassed, he and couple of other got gassed instead. He made a show of enjoying the moment before settling back down.
While he got pulled from the Gala and got a lecture about administering potions to people without their consent, something about allergies and medical conditions, he found out later that the Gala was quite fun, which was all he was after anyways.
He did have to smooth things over with his alchemy teacher, but an aerosolized version of the teacher's favorite drink and a sincere promise that this was the first and last time he'd trick anyone into coming under the effects of a potion without their consent meant that soon things were back to normal.