Royal Blood is a very important Witcher 2 Assassins of Kings Enhanced Edition quest, which must be completed in the second chapter of the game, if you sided with Iorveth, instead of Roche.
Royal Blood begins after you complete The War Council, when you arrive in Vergen.
To goal of this quest is to get one of the ingredients needed by Philippa to prepare a cure for Saskia.
Royal Blood plays a vital role in the second chapter, because while you work to complete it, you will have to make several decisions that allow you to activate other-side quests. If you make the wrong decisions, the quests remain locked and because of this you will fail to earn additional experience points for Geralt.
“Its a shame Philippa didnt demand a crocodiles tears and a sorceress simile as ingredients of the antidote for Saskia.
I believe the witcher would have an easier time obtaining either of those. As it was, he had to acquire some royal blood literally. There were only two potential donors in the area: Henselt and Stennis.”
When to start
While playing Where is Triss Merigold?
Your first objective is to see Stennis, so make sure you visit the citadel located in the eastern side of Vergen.
Talk to him and ask him to give you some royal blood. Obviously he will refuse, and you will have to wait for events to unfold.
Note: At this point, focus on the main quest called Where is Triss Merigold?, and after you bring the bandana back to Philippa, you will resume Royal Blood.
While you tell Philippa about Triss, your dialogue will be interrupted by a citizen who tells you that Stennis is in danger.
Head towards the citadel to see what happened and you will see the angry mob inside one of the halls. Dandelion and Zoltan will give you a short description of the events. Stennis is accused for trying to kill Saskia.
Iorveth appears trying to restore order, and a new cutscene unfolds, then you will get a chance to talk to Iorveth who will buy you some time to talk to the citizens.
Note: Be sure that the first character you interact with, is a peasant. Look around and talk to the one wearing a hat made of straws. Dont talk to Iorveth because you will fail to start The Walls Have Ears quest.
Hear what the peasant has to say, then start the quest you have activated immediately. Postpone the Royal Blood for now, and follow the corresponding walkthrough which explains how to solve The Walls Have Ears.
When you have enough information regarding the second chalice, a cutscene triggers and you must mediate the conflict between the angry mob and the nobles.
Note: At this point you have to make a decision. The easiest way to get the blood is to let the mob kill Stennis, but the smartest way is to stop them. Keep in mind that if you save him, you will still get the blood, and you will also complete The Walls Have Ears. Let the peasants kill him, and you wont be able to complete The Walls Have Ears, but you will get the royal blood faster.
The decision is yours, but if you wish to save Stennis, be careful during the dialogue that follows, and answer with: A lynching would be a bad idea; otherwise you will sentence him to death.
Assuming that you have managed to save Stennis, it is time to focus on the side-quest called Suspect: Thorak, and then on the main quest named Where is Triss Merigold? . The latter will allow you to find the royal blood, from another source.