I am thinking about doing an Under tale Halloween Rp. Still working on the details of it but I am thinking that Audrey Paige (perseverance) a witch tries to cast a spell and it goes wrong turning humans wearing costumes into actual monsters. Monsters are not affected because they are already monsters. Living costumes or animated objects are also a possibility. People under the effect of the spell gain abilities and weaknesses of the costume they were wearing at the time but only around the same level they are. For example, dressing up as Superman will not make you as powerful as him, but you will gain the same abilities. (For balance, up for debate.)
Corrupted wearers will try to attack normal people and force them to wear a mask to become one of them. Some corrupted wearers can infect others depending on the costume. For example, a werewolf can bite other people and turn them into new werewolves. There are ways to fix the cursed. You can use monster magic to destroy the fused costumes, defeat them in combat, talk them down by convincing them to fight it, purify them, and more. The cursed lose themselves in the costume and think they are the costume. The only way to end the curse is to find Paige's ritual circle and destroy it. It up to Frisk, Lawrence, and your Oc to help save the night.
(May edit later because I am still working on it.)