This thing of darkness Introduction
I'm Prospero. I'm a really powerful guy, and I use magic and servants to get back at people, even if it's been a really long time since they hurt me. And you know what I think?
Mark but the badges of these men, my lords,
Then say if they be true. This mis-shapen knave,
His mother was a witch, and one so strong
That could control the moon, make flows and ebbs,
And deal in her command without her power.
These three have robb'd me; and this demi-devil—
For he's a bastard one--had plotted with them
To take my life. Two of these fellows you
Must know and own; this thing of darkness!
Acknowledge mine. (5.1.267-276)
Who Said It and Where
A lot has happened in The Tempest before we get to this point. Caliban, slave-begotten-of-a-witch, has been plotting with the King's drunken butler, Stefano, and jester, Trinculo, to murder Prospero so they can rule the island.
Why do they hate Prospero so much? Well, the guy is the mastermind behind pretty much everything in the play. He used to be the Duke of Milan until his brother, Antonio, betrayed him and stole the dukedom.
Prospero was too busy learning magic in his library (arguably, not really his job). So he and his three-year-old daughter Miranda were shuttled out to the ocean in a wreck of a boat. They ended up on this island, where the ex-Duke has raised his daughter for the last twelve years.
But before you go feeling sorry for Prospero, we should tell you that he has two slaves: one is a delicate and airy spirit who was imprisoned in a tree by a witch for not being nasty enough (Ariel) and the other is the child of a witch and the Devil (Caliban). Guess who's Prospero's favorite.
Caliban is bitter at Prospero because he's treated really poorly, so he pledges to be Stefano's slave to get he and Trinculo on board. But just as the trio set out to murder Prospero, he sets hounds upon them, and the would-be-murderers run off.
Nobody messes with Prospero. He makes sure of it. He had his other slave spying on the trio the whole time. In this scene, he reveals the whole thing to King Alonso.
Prospero tells Ariel to free Caliban and his companions from the whole "being savagely hunted by hounds" spell. Alonso claims Stefano as his drunken butler, and Prospero demands that Caliban take his friends and go to work tidying up the cell, if he wants forgiveness. He's in a merciful mood today. How nice.
He lets Caliban off the hook, but not before giving him a good tongue-lashing about how awful he is. Check out how he describes his slave as "this thing of darkness." Ouch.