"A long time ago on a peculiar day, my mirror lured someone out of his bed...I do not know if the mirror was being mischevious...or malicious...perhaps it knew that the mysterious intruder had a role to play in my...project" --Yen Sid
In Disney's Epic Mickey, the sorcerer Yen Sid, (Also in the Fantasia Short "The Sorcerer's Aprentice") created a pen-and-paper world for his forgotten characters, held in an intricate model on a table, and accessible through a mirror. Mickey spills paint thinner on the pen-and-paper world, thus ravaging the world and creating The Phantom Blot.
Many years ago Yen Sid created a world for all the Disney forgotten, rejected, and retired creations. He crafted an imaginative world full of different wonders that would appeal to anyone who were to enter it. Yen Sid put the finishing touches on his world. Ignorant, of what Mickey was about to do this new world, by the time Yen Sid discovered the ruined world, caused by Mickey, created the Blot (he didn't know about Mickey's identity yet), it was too late... It is unknown what happened to him afterward, and why he did not just throw away the model, or even try to clean and fix it as the years passed.
In Epic Mickey
As Mickey successfully leaves Wasteland, Yen Sid was already there, grabbing Mickey as he comes back into the world. Mickey nervously gave him back his brush, and Yen Sid commanded him to leave his place and back to his house. As Mickey returns to his house, Yen Sid uses the mirror to give Mickey one last look at Wasteland and see the consequences of his past actions while in there.
The observant reader will notice that "Yen Sid" is "Disney" spelled backwards, a mirror-reflection, if you will. Also, the unnamed sorcerer in the film "Fantasia" was nicknamed "Yen Sid" by the studio animation department.