The Emperor's Throne is the ultimate source of power in the Infinite Ocean. When someone activates it and sits on it, power flows into them and makes them the Emperor, giving them the power to dominate the whole Magic Dimension.
The Emperor's Throne is a throne made of stone with spiny-like projections on its sides and back.
In "The Rise of Tritannus," Tritannus stole the powers of the gatekeepers of the portals that connect the ocean of Andros to the oceans of all the realms of the Magic Dimension, namely those of Andros and of Earth and he vowed to do the same to all the members of their kind so that he will be able to enter the Infinite Ocean and activate the Emperor's Throne, which will allow him to conquer the whole Magic Dimension.
In "The Emperor's Throne", after entering the Infinite Ocean with the Trix, his mutants, and Daphne as his prisoner, Tritannus tried to activate the Emperor's Throne but failed and tried to damage the Throne with his trident, causing the Throne to attack him, and had Daphne imprisoned when she warned him of the dangers of the Sirenix he stole from her.
Because the attack by the Emperor's Throne drained him of his powers, he had to go to Earth with the Trix to refuel his powers. However, the Winx found them, and when Tritannus and the Trix fled into the Infinite Ocean the Winx followed them, surprising the Trix and Tritannus. When the Winx tried attacking Tritannus, he showed them Queen Ligea, King Neptune, Nereus and Tressa turned into mutants, and told the Winx that they will have to destroy Aisha's family to destroy him, and he ran away with the Trix to activate the Emperor's Throne.
Later, when inspecting the Throne to find out why it was not working, Tritannus found out that the legs of the Throne were broken and decided to steal the seals of the Pillars of the Infinite Ocean and fit them where the leg of the Throne was broken to repair it and activate it. When he took the seal of the Pillar of Light this caused an eclipse in the whole Magic Dimension that made all the realms unstable.
When Tritannus was about to place the seal of the Pillar of Light in the broken leg of the Emperor's Throne, Daphne told Darcy and Stormy to stop him, because if he succeeded he would release a power that Tritannus himself wouldn't be able to control, and would destroy the Infinite Ocean and everything and everyone, including Tritannus himself and the Trix.
When Tritannus placed the seal in the Throne, its powers attacked him and drained him of his toxic powers, making him weak enough so that his hold on Daphne became weak and she was able to contact Bloom to warn her about Tritannus' plans and tell her how to stop him.
With the seals of the Pillar of Light and of the Pillar of Balance and Aisha's Sirenix powers, Tritannus activates the Emperor's Throne and gains the Ultimate Power of the Infinite Ocean. However the power turns him into a monster and he attacks Icy. Tritannus is later defeated by Nereus and Bloom.
While trying to activate the Emperor's Throne, the Trix accidentally evoke Politea, who is still cursed and is ready to ally with the Trix. She tells them that, to activate the Emperor's Throne, they need to free Tritannus from Oblivion but that they need the vital force of a king for that. The Trix kidnap Sky and free Tritannus, who becomes their ally again to get the Pearl of the Deep. Later, she meets the Winx and uses her powers against Aisha and Bloom causing them to fall to the floor of the Emperor's Throne. Then she takes away Stella's Sirenix powers which traps and tangles her in it. Later on, she pollutes Domino's ocean by using the Pearl of the Deep. She is then destroyed by the Winx never to be seen again and with her gone Domino's ocean become clear once more.