The first start Tombs of the Fallen in Assassin’s Creed Valhalla is Manius’ Sanctum. You can find it right next to your settlement after initially investigating why the animals are going wild. After you approach the Tomb and enter it, the protective barrier around it will fall, giving you access to the Tomb. Here’s what you need to do to complete Manius’s Sanctum.
When you initially make your way into the Tomb and reach the bottom, you’ll be blocked with a breakable rock wall, or you can choose to take the path to the right. If you wish to break the wall, to the right, there will be an oil jar you can grab at the top of a platform to use on the breakable wall. Behind it will be a handful of opals and small items for you to pick up, but the bigger prizes in the Tomb are off to your right.
When you proceed to the right, further down the path, you’ll eventually reach a room with a large golden door, two pressure plates on the floor, and a breakable wall on the right side.
Use a melee attack or an arrow to destroy the breakable wall, and behind it, you’ll find a large block that you need to drag out to the center room and place on the pressure plate. Once the block is on the pressure plate, you need to do the same for the other.
Scale the wall on the left side of the room, and you’ll find a large lever. Pull it, and a small ball appears on the ground. You’ll need to pick it up, bring it back to the center room, and place it on the second pressure plate. When you have both items on the pressure plate, the initial gate should open, allowing you access.
When you reach the inner chamber, you’ll discover a larger opening at the bottom. In the backroom, you’ll find all of the Tombs of the Fallen that you can find in England, providing you exact locations for where you need to go to finish opening up this chamber.
Once you’ve explored the Tomb, collect the key at the back of the chamber, and continue through the final door. There will be a chest protected by a bobby-trapped floor plate that you can obtain, collecting the Fallen Tomb cloak.