If thou more murmur’st, I will rend an oak
And peg thee in his knotty entrails till
Thou hast howled away twelve winters. (I.ii)

In response to Ariel’s concern that Prospero will not grant him freedom for his faithful service, Prospero reminds Ariel of how he saved him from the witch Sycorax and then issues this threat. The violence of this threat illustrates both Prospero’s bad temper and his domineering nature. Clearly, Prospero does not tolerate disobedience from his inferiors, and he relies on threats of cruelty to keep those under his command in line.

If thou neglect’st or dost unwillingly
What I command, I’ll rack thee with old cramps,
Fill all thy bones with aches, make thee roar
That beasts shall tremble at thy din. (I.ii)

Prospero directs these harsh words toward Caliban, who has just resisted his command to fetch sticks for a fire. Prospero once again demonstrates his willingness to use (and perhaps indicates his history of using) magic for cruel purposes. These words also recall something Prospero said earlier in the scene, when he reminded Ariel of the torment he suffered under Sycorax’s rule: “Thy groans / Did make wolves howl and penetrate the breasts / Of ever angry bears” (I.ii.). Prospero’s implication that he would make Caliban suffer just as Sycorax made Ariel suffer suggests that little separates Prospero from that “foul witch” (I.ii.).

If thou dost break her virgin-knot before
All sanctimonious ceremonies may
With full and holy rite be ministered,
No sweet aspersion shall the heavens let fall
To make this contract grow, but barren hate,
Sour-eyed disdain, and discord shall bestrew
The union of your bed with weeds so loathly
That you shall hate it both. (IV.i.)

With this long-winded sentence, Prospero informs Ferdinand as to what will happen should his soon-to-be son-in-law attempt to have sex with Miranda before they get married. Unlike with Prospero’s other threats, which indicate just how capable of cruelty he is, this threat illustrates his protective nature. When he says that premarital sex will sow “disdain” and “discord” in their marriage and render their relationship “barren,” he does not really mean it. Instead, Prospero uses such strong words so that Ferdinand understands how precious his daughter is to him.