This is the first quest of the Assassin's Creed Crossover Stories DLC and you should receive it automatically as soon as you make some progress in the main storyline of the game (Valka must live in your settlement).
An odd, young woman has arrived at Ravensthorpe and has set up camp near Valka's Hut.
Head your steps towards Valka's hut in Ravensthorpe. There, on a nearby hill, you will run into Edyt.
When you start talking to her then after a while Valka will join you and introduce you to what the problem is and what you may face.
When you are ready to go to the island then inform Edyt about it and then you will automatically set off.
When you get there Edyt will inform you where you should start your investigation. This quest will end and you will receive the new one.
