After you complete the first half of the “Where Is Triss Merigold” quest and obtain Triss’ bandanna, return to Philippa. Hand her the bandanna. She will be able to look into finding Triss for you, but you will still have to finish obtaining all of the items necessary for her to cure Saskia. The last of these items is the blood of royalty. Head outside into the town square to find a violent scene erupting. The locals will have seemingly discovered someone responsible for poisoning Saskia. Turns out it was Stennis after al. The local militia will be attempting to lynch him, while the nobles will be trying to protect him from this fate. You will have to act quickly or risk failing the mission.
You can only talk to a limited number of people now that this phase of the quest has begun. First, speak with the dwarf named Halldor Halldorson. Then, speak with the townspeople of Aedirn. After this, head over to speak with Iorveth. He will give you his take on the situation. After speaking with him, turn around and head over to Dandelion and Zoltan to hear from them. They will give their opinions on what they think should be done with Stennis. Speak with Johlan nearby as well to hear another opinion. Finally, speak with Stennis. You will have to go through his guards first, though, so use Axii during your conversation to persuade them into letting you through.
You will now be tasked with choosing Stennis’ fate. You can either have him lynched, or put him on trial for his crimes. Take into consideration all of the different opinions that you have heard so far. Also take into account the repercussions of your actions. If you lynch him, you can simply collect the blood from his corpse. If you choose to save him, you will have to return to the Where Is Triss Merigold quest and obtain some blood from Henselt during the latter portion of this quest. Stennis will be sentenced to prison if you make this choice. For these reasons, it will be easier to simply sentence him from death and collect the blood from the corpse in front of you. No matter which option you choose, you will be rewarded with 1,500 experience points and you will be freed to pursue other quest lines. If you have the blood now, return it to Philippa. If you saved Stennis, head out to complete Where Is Triss Merigold now.