Reward: The Daichi
To unlock this tomb you first have to complete the quest "Cullis Gate", after completing that quest Brightwood Tower will come up for sale, you do not have to purcahse this Tower to start Archon's Knot Tomb.
If you meet the above requirements, head to Brightwood Tower where you will fight the same enemies as before (with Garth and Hammer). Go to the second level of the Tower where you entered the Cullis Gate, next to the Cullis Gate you will see a deviation where a statue used to be. Walk up to it and vault down.
You will dive down into some water, and behind you is a gargoyle. Proceed where you will come up to a locked gate, no worries, to your right is a slowing switch, shoot it to unlock the door. Continue and you will notice the next door seems quite unreachable. To your left is another flowing switch, shoot it and it will go towards the door.
Hit the switch at the end to unlock the door.
Follow the path up the stairs and you will notice a switch that you have to hit is locked up in a cage next to a statue, dont worry about it yet and keep going where you will find another room with spiked floors and a pressure plate. Step on the pressure plate and the statue will move, go back down and do a Bloodlust Roar towards the statue to free the switch.
Go back upstairs and hit the switch, keep hitting it and it will show you what path to take, when you get to safety hit it one more time to unlock the gate. When the gate is unlocked the spikes are no longer active so you can grab that silver key. Leave the tomb, your dog will alert you of some treasure hidden around the corner on a platform. Voila, The Daichi is in the chest there. If you want you can dive down and swim around to that island in the middle of the water, inside the building is a chest.