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The Shadow Realm is a distant, barren and desolate place located in space, far across the mystic void. It is not possible to enter or leave it via normal means.
Many years ago, for his evil deeds, the Shadowmaster was banished to the Shadow Realm by a young Claudus, who was the Lord of the ThunderCats at that time. Using the Sword of Omens, he captured the Shadowmaster in an energy bubble and transported him to the Shadow Realm.
The Shadowmaster remained in his rocky place of exile for many years, developing his powers and even building a rocky fortress for himself. Using his magic, he was able to observe all that was happening on Thundera. When Thundera was on the brink of destruction and all of its inhabitants were fleeing the planet, the Shadowmaster used his magic to capture Claudus. As that day coincided with an eclipse of the Dark Moon of Plun-Darr, the Shadowmaster was able to open a small portal that enabled him to bring Claudus to the Shadow Realm.
Many years later, another eclipse of the Dark Moon allowed the Shadowmaster to send a message to the Ancient Spirits of Evil. He strikes a deal with them in which they would free him from the Shadow Realm if he succeeded in destroying the ThunderCats. Using Claudus as bait, the Shadowmaster lured Lion-O into the Shadow Realm in order to kill him, but the young lord proved to be a better fighter and defeated the Shadowmaster as well as freeing his father.