There’s a book of knowledge in Dover Fortress of Cent in Assassin’s Creed Valhalla.
This book will give the ability of Thorn of Slumber.
Here’s how you can get the book of knowledge in Dover Fortress in AC Valhalla.
To begin, head to the location shown in the map, marked by the book symbol.
How to get the book of knowledge
Before you can get the book, you need a key.
Enter the tower and climb all the way up.
You will find a key at the top, overlooking the sea.
Head back down and you’ll now be able to unlock the room which holds the book.
Collect the book to get the Thorn of Slumber ability.