On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the Falling Stars side quest.
Rewards: 1,200 XP.
In the place marked in the image above, you will meet some crying women - the reason for their whining is the cancellation of an announced performance. Read the two documents that are nearby: on the small altar and on the wall. You now need to help three people to make the show happen.
Head near St. Paul's Cathedral and you'll meet a woman (Kitt) playing a strange instrument, who hasn't been here before. From your conversation with her, you will learn that the woman is in debt. Give her 50 pieces of silver so she can pay it back.
Behind the city walls, in the place marked in the image above, you will meet another woman. Ysane has a serious problem - she can't write poetry without her muse, who clearly doesn't love her anymore and has left her.
Give her a short rhyme on any topic to teach her another way to find inspiration and write lyrics.
In the place marked on the image above, you will find caged Norman men.
Among them will be Cynewulf, previously mentioned by the women. Free him from his bonds and give him back his freedom.
Once you've helped Kitt, Ysane, and freed Cynewulf, head to the plaza to see if they'll perform the previously planned show together. The mission will come to an end.