Zant is a Twili. He is a server to the Royal Family and when the King of Twili passed away he thought that he would gain the throne but instead the Twilight Princess Midna got the throne. He was mad and then he met Ganondorf who had been sent to the Twilight Realm due to the Ancient Sages. Ganondorf takes his power and puts it into Zant partly controlling his actions. Zant then overthrew Midna and sent her into Hyrule. He took over the Twilight Realm and turned every one into his own army. Then he went into Hyrule and slowly turned parts of it into Twilight. He went to Hyrule Castle and due to his enormous power Princess Zelda had to give up her throne to him. But as he was taking the last part of the Light World (Hyrule) into Twilight Link a boy from Ordon Village turned into a wolf Midna saw this and joined up with him. Zant watched and waited as they found and destroyed the areas of Twilight then finding the pieces of the Fused Shadows. When they found the last one Zant came and took them from Midna then took away all of Midna's power while leaving Link as a wolf. Zant then went to Hyrule Castle and Ganondorf used his power to turn himself back into Ganondorf.
Zant sa

Zant with his Shadow Assasssins

He ordered Zant to go back to the Twilight Realm Then Zant went back to the Twilight Realm and on his way destroyed the Mirror of Twilight leaving shards in sacred places all around Hyrule. Zant waited in the Twilight Realm for Midna and Link to piece together the pieces of the Mirror of Twilight. For he knew that they would come because it was all a part of Ganondorf's Plan. While Link took the time to find the pieces of the Mirror and go and Fight Zant, Ganondorf would have taken over Hyrule Castle along with taking Princess Zelda.
Zant no helmet

Zant without his Helmet

But the one thing that Zant didn't foresee was Link beating him and getting back the Fused Shadows. Then Midna using the Fused Shadows to kill Zant.