The Phoenix Rising side quest in Hogwarts Legacy becomes available during the Prepare for Your O.W.L.S main objective. Make your way to the Room of Requirement and speak with Deek. He’ll mention a Phoenix is nesting nearby. Set out in search of the mythical bird by heading to the northeastern part of the Poidsear Coast to find the Phoenix Cave Mountain. Head inside then follow the path through the cave. Eventually, you find the Phoenix. Follow it while taking out the Dark Wizards hunting it. When you reach the end, the Phoenix will let you rescue him. Return to Deek to complete the quest.