but he always seems to find one more task for Ariel to undertake before he truly sets him free.
At least Ariel has the promise of freedom at some time in the future, but Caliban is given no such promise, and he's held in servitude by Prospero throughout the play.
Prospero tells Ariel, “I shall miss thee, / But yet thou shalt have freedom” (5.1.103–104), but, as usual, Prospero finds another task for Ariel to accomplish before he finally sets him free. Prospero sends Ariel to release the ship’s master and boatswain from sleep in the hidden ship and to “enforce [bring] them to this place” (5.1.108). Ariel hurries away with yet another promise of freedom ringing in his ears.
Ariel returns with the ship’s master and boatswain. Prospero commends him on his diligence, and again promises, “Thou shalt be free” (5.1.280–281). Then Prospero sends Ariel off on yet another task, this time to release Caliban, Stephano, and Trinculo from Prospero’s spell and bring them before him.
Within seconds, Ariel returns with Caliban and his companions. Caliban sees Prospero dressed in his attire as Duke of Milan and is fearful of him and what he might do.
CALIBAN. How fine my master is! I am afraid
He will chastise me. … I shall be pinched to death. (5.1.305–306, 321)
Caliban escapes being pinched to death, but Prospero says nothing at all to Caliban about setting him free. Prospero tells Caliban only how to procure his pardon for conspiring with Stephano and Trinculo to kill him.
PROSPERO. Go, sirrah, to my cell.
Take with you your companions. As you look
To have my pardon, trim it handsomely. (5.1.338–340)
Caliban agrees to do as he’s told and “seek for grace” from Prospero (5.1.342); then, Caliban exits with Stephano and Trinculo and is not seen again in the play.
Prospero intends to spend one more night on the island, then set sail to Milan in the morning with his daughter, Miranda, and everyone who was supposedly shipwrecked on the island by the tempest Prospero caused at the beginning of the play.
The question arises as to whether Caliban will go with Prospero on the ship to Milan and continue to serve him or if Caliban will be left on the island.
Caliban will have no freedom if he goes with Prospero to Milan. If Caliban is left on the island, he’ll enjoy the freedom of the island in his role as the rightful ruler of the island, a role that Prospero usurped from Caliban in the same way that Prospero’s brother, Antonio, usurped Prospero’s role as the rightful Duke of Milan.
There’s no mention in the play as to which fate awaits Caliban the next day.
As for Ariel, Prospero finally, finally grants him his freedom, but only after Ariel insures “calm seas, auspicious gales / And sail so expeditious” (5.1.365–366) that the ship will quickly reunite with King Alonso’s royal fleet, from which his ship was separated in the tempest.
PROSPERO. [to Ariel] That is thy charge. Then to the elements
Be free, and fare thou well! (5.1.368–369)
Of course, Prospero has all night to think about more tasks for Ariel to undertake before the ship sails away in the morning.