Before the Isle of the Lost
Yen Sid was a powerful sorcerer who had an apprentice, Mickey Mouse. When Yen Sid left, his apprentice later took his hat to learn magic from himself, after Mickey brings a broom to life and to do his job. It ended up filling the whole room with water. Yen Sid returns and stops the chaos and realizes that it was his apprentice's fault and sends him back to work.
Yen Sid resides on the Isle of the Lost as the science professor of Dragon Hall. He was sent on behalf of King Beast to teach the next generation of villains how to live without magic and using science instead. Despite leaving Auradon, Yen Sid takes the obligation in stride and eagerly teaches the students, good or bad.
Yen Sid founded the Anti-Heroes Club after the four villain kids settled down in Auradon. When unseasonal earthquakes and storms occur around Auradon, Yen Sid sensed a powerful source of magic underground, and had his kids send messages to lure the four back. Once returned, Yen Sid told them what he'd learned and urged them to find the Talismans before they land in the wrong hands, while also giving them important advice.
- His name is "Disney" spelled backwards.
- He is the sorcerer from Disney's Fantasia.
- He volunteered to live on the Isle of the Lost to teach the kids the magic of science at Dragon Hall.
- He is one of the characters not to talk in their film debut, the first being Dopey, the second Gideon, the fourth Chernabog, the fifth Dumbo
- With his confirmed appearance, it would revealed that the character Mickey Mouse (known as Sorcerer Mickey) exist since Mickey was his apprentice
- The apprentice job was originally given by Dopey.