This is the first quest of the game's sixth chapter. After defeating the Crowcaller - which takes place during the "Toward the Nest" quest - Kliff will return to Pywel, near the camp on Howling Hill.
With the incidents involving White Crow resolved, long-held tension has finally given way to fatigue. Head back to Howling Hill to get some well-earned rest with your comrades.
Just ride into the camp and approach the main tent to trigger a cutscene.
You don't need to do anything - the entire conversation takes place during the cutscene. A soldier will inform you about another member of the Greymanes found in Calphade, and that the Marquis Serkis wants to speak with you about it.
Simply head to Castle Hernand and approach the door on the upper floor - this will trigger a cutscene, after which this quest will end and the next one will begin - To the Battlefield.
A messenger brought word that Marquis Serkis requested your presence. You headed to Hernand Castle to meet Duke Celeste, who delivered the news that Calphade is under attack by the Black Bears and a Greymane is caught in the middle of it. Set out immediately to save your comrade.