He is a ghost who lives in the castle attic along with his ghost bat friend, Boo. Upon Halloween, he planned to host a Ghostly Gala for his ghostly friends and family but unfortunately, Sofia's Halloween party was on the same night.
In order to host his Gala, he tried to scare everyone out of the castle as he believed everyone would freak out if they saw his ghostly friends. Sofia kept interfering in his attempt to scare everyone but didn't realize that Sir Dax only gets to see his friends and family every one hundred years on Halloween night, and if he doesn't get to host his Ghostly Gala, he will not get to see them for another century.
Things soon went out of control when Sir Dax used bats to frighten everyone away. Although he succeeded, he couldn't get rid of the bats. With help from Sofia, he was able to scare them out of the castle and in gratitude for helping him, he decided they could have both their parties.
Sir Dax got to host his Ghostly Gala and Sofia got to host her Halloween party, while everyone believed all of Sir Dax's ghostly friends were just Halloween tricks made by Sofia.