Who imprisoned Ariel in the tempest?

In Shakespeare's play "The Tempest," Ariel, a spirit, is imprisoned by the witch Sycorax in a cloven pine tree. After Sycorax's death, Prospero, who is Sycorax's former master, frees Ariel from the tree and enlists his help in carrying out his plans for revenge and reconciliation on the island. Ariel serves Prospero throughout the play, seeking his ultimate liberation in return for his service.