Stuck is a boss that drops Tales of The Universe, spawns every 30 minutes and has 1/10 chance to drop. It is an extremely hard boss, as it can spam moves onto you. Timestop, or percentage stands are HIGHLY recommended for this.
"Stuck" is actually one of the developers of the game known as "StuckDuck".
The boss theme is Genshin Impact OST - Taishan Mansion.
Stuck spawns near the Colosseum and a message will appear when it does in the chat.