In Twilight Princess, the usurper king of the Twilight Realm is Zant.
Zant is a Twili sorcerer who overthrows Midna, the true ruler of the Twilight Realm, after receiving power and encouragement from Ganondorf. He takes the throne and transforms the realm’s peaceful inhabitants into shadowy monsters, spreading twilight into the world of Hyrule. His title and actions make him the game’s central usurper rather than its ultimate villain.
Zant is sometimes mistaken for Ganondorf because Ganondorf is the force behind the takeover. Ganondorf manipulates Zant and grants him supernatural power, but the two characters are distinct: Zant is the Twili usurper king, while Ganondorf is the imprisoned Gerudo demon king who uses him as an agent.
After Link and Midna confront Zant, he is defeated and the usurped throne is restored to Midna. Zant’s collapse also reveals the limits of his power and his dependence on Ganondorf. The game’s final conflict then shifts from Zant to Ganondorf, who remains the ultimate antagonist.