The Great War was a war waged between the forces of light and darkness, decades before the events of the Skylanders Academy TV show. Though long over, it is the backstory to the show's events and characters, and its consequences still threaten Skylands and are the cause of events from all three seasons.
Though the three sorcerers and sorceress Eon, Kaossandra and Strykore were initially a powerful trio training their magic abilities, Stykore grew greedy and learned various illegal spells, turning to the side of darkness. With Strykore and other villains rising in power as he devoured the light energy of creatures, the Core of Light was threatened after its great dragon guardians were corrupted by the evil wizard's magic. The dragons' protector, Master Eon, was then forced to seal the dragons away by using powers far beyond his control, sending them to parts unknown to save the Core; while Strykore was betrayed by Kaossandra, who realized the true danger of destroying the universe's balance, and sealed him in the Book of Dark Magic with Eon's help after the wizard could not bring himself to kill his own brother.
Seeing the threat the Core of Light suffered even after Strykore's defeat, Master Eon put it inside The Isle of the Citadel that he himself built, and for a long time used it as a fortress of solitude to think of what to do next to protect Skylands.