Usurper King Zant was originally a Twili (a race whose dabblings in dark magic got them sealed away in a shadow realm) whom Ganondorf tricked into serving him. Ganon persuaded him by giving him the means to conquer the Twilight Realm and begin an invasion of Hyrule, and Zant began treating Ganon as a god. To conquer Zant, Midna, the true ruler of the Twilight Realm, partnered up with Link and they both began collecting Fused Shadows, artifacts of power that the goddesses had hidden away. Midna hoped to defeat Zant to regain both her original form and her birthright as the Twilis' ruler. Though mortally wounded in his battle with Link, Zant reveals the curse he cast on Midna to be dependent on Ganondorf and is killed by a furious Midna soon after.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.