Royal Blood

Prelude to War: Kaedwen is a quest in Chapter 2 of The Witcher 2: Assassins of Kings.

Walkthrough
You need royal blood to make the antidote for saskia. This can come from the prince or from King Henselt.

If you let the mob lynch the prince, you get the blood right then. If you let him live, you'll later have to convince Henselt to give it to you.

In chapter 3, it is revealed that the prince did indeed order Saskia's poisoning. If you had let him live, he will become King.