What stylistic devices are used in this quote from Watership Down: "When they catch you, they will kill you; but first, they must catch you."

This quote comes from the narrator or the character of Frith and is said to Hazel. The full quote that is said to Hazel is as follows:

All the world will be your enemy, Prince with a Thousand Enemies. And whenever they catch you, they will kill you. But first, they must catch you, digger, listener, runner. Prince with a swift warning. Be cunning, and full of tricks, and your people will never be destroyed.

I am not too sure that the quote you have given uses any stylistic devices apart from the various ways that the narrator names Hazel using such titles as "Prince with a Thousand Enemies" and "Prince with a swift warning." The naming of this quote seems to emphasise the danger that Hazel is in whilst at the same time offering hope of the various attributes that Hazel possesses to help him escape that danger. Note the way that the narrator addresses Hazel as "digger, listener, runner," emphasising those very qualities that will help him to escape and survive. The quote therefore captures the very real danger that Hazel faces, but at the same time the hope that he has because of his personal attributes that will allow him to survive that danger.

What is the exact quote and context in Watership Down similar to: "When they catch you, they will kill you; but first, they must catch you."

That quote is part of a larger story, which is below. It is part of the mythology of why the rabbits are hunted by so many animals.

"Long ago, the great Frith made the world. He made all the stars, and the Earth lived among the stars. He made all the animals and birds, and at first, he made them all the same. Now, among the animals in these days was El-Ahrairah, the prince of rabbits. He had many friends, and they all ate grass together. But after a time, the rabbits wandered everywhere, multiplying and eating as they went.

Then Frith said to El-Ahrairah, 'Prince Rabbit, if you cannot control your people, I shall find ways to control them.' But El-Ahrairah would not listen. He said to Frith, 'My people are the strongest in the world.'

This angered Frith, and he determined to get the better of El-Ahrairah. And so, he gave a present to every animal and bird, making each one different from the rest. When the fox came, and others, like the dog, and cat, hawk, and weasel, to each of them, Frith gave a fierce desire to hunt and kill the children of El-Ahrairah.

Your people cannot rule the world, for I will not have it so. All the world will be your enemy, Prince with a Thousand Enemies, and whenever they catch you, they will kill you. But first they must catch you, digger, listener, runner, prince with the swift warning. Be cunning and full of tricks and your people shall never be destroyed."