A spirit of the air and guardian of innocence. He was enslaved to the witch Sycorax, who overtasked him; and in punishment for not doing what was beyond his power, shut him up in a pine-rift for twelve years. On the death of Sycorax, Ariel became the slave of Caliban, who tortured him most cruelly. Prospero liberated him from the pine-rift, and the grateful fairy served him for sixteen years, when he was set free.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894