In Act IV, Scene 1, Stephano, Trinculo, and Caliban go to Prospero's hut with the intent to kill him and take over the island. Prospero and Ariel are ready, however.
Prospero commands the spirits to attack the men. Spirits take the shape of dogs and hunt the men. After they run off, Prospero commands that the spirits not only continue to chase them but also torture them by giving them convulsions and cramps.
