A Favor for Radovid is a short main quest in The Witcher 3 that starts the moment Geralt walks out of Whoreson Junior’s estate at the end of Get Junior. Redanian soldiers are waiting, and they take him to King Radovid V aboard his ship in Oxenfurt harbour.
Radovid helped Geralt find Junior, and now he wants the favour returned. His request is very much in character: find Philippa Eilhart, the sorceress who was once his mentor and who he now hates with a particular fury, and bring her to him. Geralt knows full well that kings always need someone to do their dirty work, and this conversation shows just how unstable Radovid has become, with his chess metaphors, his paranoia and his obsession with the mages.
The conversation itself is short, but it has long-reaching effects. It starts Redania’s Most Wanted, the questline that ties Geralt, Dijkstra, Roche and Philippa together and eventually decides the fate of Radovid and the North.