On this page of the Assassin's Creed Valhalla Siege of Paris guide you will find a detailed walkthrough of the Fire from Heaven main story quest.
Reward: 1,700 XP.
Meet with Sigfred
This quest begins in Saint-Merri. In the bell tower you'll discover a trail of blood, leading to the very top. The path upwards isn't difficult and requires climbing two ladders. You'll reach the top of the tower and Sigfred. A cutscene will start.
You'll be following Sigfred for a while. While doing so you'll talk to various companions and rally them to support you in the upcoming fight.
Eradicate the Frankish soldiers
You will face off against a large group of soldiers. They shouldn't cause you much trouble, however. You're surrounded by allies and resources you can use to heal.
After eliminating all enemies talk to Sigfred, finishing the quest.