Caliban, summoned by Prospero, enters cursing his master:
As wicked dew as e'er my mother brushed
With raven's feather from unwholesome fen
Drop on you both! A south-west blow on ye
And blister you all o'er!
Prospero then threatens him with cramps if he doesn't behave himself, and Caliban then makes his famous claim of sovereignty over the island:
This island's mine, by Sycorax my mother,
Which thou tak'st from me.
What has soured Prospero and Miranda against him? It seems they treated him very well, until he tried to rape Miranda, as Prospero says:
I have used thee,
Filth as thou art, with human care, and lodged thee
In mine own cell, till thou didst seek to violate
The honour of my child.