This guide will let you know how to unlock and complete the quest Cassivellaunus Tomb for Tombs Of The Fallen in Assassin’s Creed Valhalla.
First, you will need to go to Sciropescire from the map. When you get there, you have to use your bird to help you find the tornado.

You will need to use your boat, follow the path, and dive underwater when you reach the end. As shown in the image below, you have to climb out of the water and then use the staircase.

After that, you have to jump down towards the columns, then go to the wooden bridge and follow the path. Once you are near the boat, you have to interact with the lever; then, you must grab an object and take it with you.

Now you need to jump on the wooden structure, go on top of it, and find a rock. So you have to grab that rock and place it on the steel stand under it so that it will sink in the water, as shown in the image below.

You need to jump back into your boat and follow the path out. When you reach the end, you have to grab the metallic round material and step out of the boat. Now you need to place it in the middle of the pressure plate.

The next thing you will need to do is go under the waterfall, and there will be a crack in the wall; you have to go through that and go inside. Go through the water, and you need to grab the golden orb there, as shown in the image below.

Now you need to use your bow and arrow and shoot an arrow at an obstacle that will clear your path and then go through it. After moving forward, you have to shoot an arrow at the next obstacle and make your way clear.

You will need to keep going and follow the path. You can also use the hawk vision and navigate the lever. Now you need to shoot an arrow at it, which will help you clear the path and keep moving forward, as shown in the image below.

Once you reach the end, place the golden orb on the pressure plate. So now you need to go to the right side and climb to the top of the waterfall. When you get there, you will see a huge crack in the wall, so you have to get inside through the crack, and there you will see that the floor is made of wood, so you have to use your melee weapon and break it as shown in the image below.

You have to push the stone weights out of the doorway and then click on it and grab the key. You have to unlock the door with it, take the stone weight, and place it on the pressure plate, as shown in the image below.

Now you will see that the tomb’s gate is open; you can easily get in the tomb and have chests.