This page of the Assassin's Creed Valhalla guide has a detailed walkthrough of The Letter quest.
Hytham has received a strange letter and wants to reveal its contents to Eivor. You will get access to this quest after eliminating all members of the Order (except Grand Maegester) and after conquering England (Hamtunscire storyline).
Speak to Hytham
Approach Hytham, whom you will find by the longhouse. The man will inform you that he has received a rather peculiar message. Eivor receives an invitation to the Aethelnay settlement, and the sender of the letter is a Poor Fellow-Soldier of Christ.