swordsandroses: You can obtain a key from Thorak, if you completed the nightmare quest from the harpy den.
I am puzzled as to what chest does it open and where is it. If anyone got a clue please share
*MAJOR SPOILERS*
This is correct. If you have completed Baltimore's Dream and actually killed Thorak in the process (you will get a cutscene, once you find the door in the mountain that the 'harpy' key unlocks), you will get a key from his corpse. This key can be used during Royal Blood and after speaking to the cup maker (who reveals that the only other person that could attempt to make a unique cup is Thorak) to open a small chest, inside Thorak's House. Inside, you'll find a document stating that it was actually the priest that ordered the poisoned cup. The journal entry implies that he may have had some personal issue with Saskia (he considered her unholy or something, if memory serves), however there is no mention of whether someone else might have asked him (or even forced him) to poison her. This will also complete a quest called 'Suspect : Thorak'.
This is as far as I've gone into 'Royal Blood'. You are still allowed one conversation I believe (assuming that you went to speak to the servant right away at the start of Royal Blood and ignored everyone les), before the crowd intervenes and you are forced to make a choice and conclude the quest. Funny thing is, although both Suspect Thorak and the quest you get for the cup (the name eludes me right now) appear as completed, once you finish chapter 2, they will appear as failed for some reason (perhaps because I chose to spare the prince ? who knows).
As to who is actually responsible for the poisoning, it is a bit confusing. Once you finish chapter 2 and acquire Philipa's book of poisons and antidotes inside her house, the journal entry CLEARLY states that Philippa was the one who poisoned Saskia, in order to be able to use the antidote as an excuse (she knew that she was a dragon all along) to control her (via the rose petal). The problem with this is that later on, on chapter 3, you get a cutscene about the prince, refering to him as a 'poisoner'. So, which one is it ? Right now, I'm going under the assumption that there is a bug somewhere (or my perception of the situation is somehow flawed :p).