On this page of the Assassin's Creed Valhalla Siege of Paris guide you will find a detailed walkthrough of The Missing Queen main story quest.
Investigate the Church of Saint Taurinus
This mission begins right after completing the Majesty in the Dark quest. You need to head to Evreux.
There you need to reach the church - it's easy to locate, as it's the biggest building in the area and has a cross on the roof. Your further objectives are:
- Taking a seat on the church bench and attending the mass;
- Talking to a woman standing by the pulpit.
This will begin the Sister of Sorrow quest.