Bahl'Uun is the eldritch lord of parties. He is composed entirely of balloons that people let go of outside. As they float into the cosmos, they join together and form the ever-shifting body of Bahl'Uun.
It is said that seeing him will make your sanity go 'POP'.
His only appearance in film is when he is summoned by an elderly man in 'UP' to help him escape from his boring life.