The Realm of the Past is a realm that was created by the Great Wizard. It is a realm that stored many worlds and people that have faded away with time. The Great Wizard created this realm as a means to remind himself of the terrible tragedies that he was unable to prevent. With each world and civilization that was stored there becomes more painful for him. However, it has served as a realm to store worlds that the Mystics were unable to save, but rather extracted their essence and stored them away before the worlds themselves were obliterated.
One such realm is the Realm of Spira, which barely survived the Rifter due to an immeasurable amount of chaos and corruption that made it easily susceptible to the Rifter. The Realm, itself, was not destroyed by the Rifter but was greatly damaged beyond salvation. In order to prevent the realm from disappearing completely, the Teenzie Guardians collected essences of its people and many of its places and stored it in the Realm of the Past before it was obliterated completely.
Past events are also kept in the Realm, such as events that have occurred during Super Mario World, Yoshi's Island and Ocarina of Time; a way to store what had happened around those times. Past events that the main characters of The Ultimate Story were also stored there, serving as a interactive memory of each of their past journeys.
The Realm of the Past have served as a number of important events within the story.