Reason of State is a side quest in The Witcher 3 that takes place in Novigrad and it is highly recommended that you are at least level 30 before taking on this quest. In addition, Reason of State will only become available after you have finished the rest of the quests in the chain, namely, An Eye for an Eye, A Deadly Plot, and Redania’s Most Wanted, as well as acting favorably towards Djikstra during quest "Blinding Obvious" which was part of Final Preparations.

Reason of State Walkthrough

The first part of the quest involves meeting Dijkstra, Roche,  and Thaler in a portside warehouse. Dijkstra explains his plan for assassinating Radovid. After the discussion, leave the warehouse;  a wild Philippa will appear. Talk to her; she admits to eaves dropping. She gives Geralt Radovid's fathers ring to help him convince Radovid that he indeed has found Philippa.

Head towards Radovid's ship; board it and speak with Radovid; giving him his father's ring which will convince him you are telling the truth. Radovid will leave the ship with all his men; once at the location you have described; Radovid orders his men to kill the you; try to persuade him to let you live! Roche, Ves, and the other Termerian guerillas arrive in a timely fashion to save your bacon!

Kill the Redanian soldiers around you, and follow Roche in pursuit of Radovid. You will be opposed by numerous Redanian soldiers, and witcher hunters, as you follow Radovid. (Note: If you get bugged and are unable to interact with Radovid, this may be because you used Axii in these fights.)

After Radovid's minons are killed; he tries to flee, but finds his way blocked by locked doors and gates.  A door opens suddenly; providing a way out for the panicky monarch; only it precipitates a fatal encounter with Phillipa.

The next cutscene will be of Geralt, Roche, and Ves arriving to meet Thaler, to inform him the assassination was successful. Thaler tells them he's previously negotiated an armistice/truce between Redania and Nilgard; with limited autonomy for Redania, Termeria, and Aedirn.  Dijkstra arrives; he announces the Nilfgard/Redanian pact will be abrogated in favor of his ascension to the leadership of Redania and the other northern kingdoms. Dijkstra's seizure of power includes killing Thaler, Roche, and Ves. You may side with Dijkstra, or Roche. If you oppose Dijkstra, you will have to fight and kill him and his henchmen.

Whoever's side you you took; the quest will conclude.