After bumping into the tower seal, you are told to fetch the Testament of the Dragon King. To do so. You will have to follow the maps below.

First hit the right path from Keepers’ Battle Observation Deck, follow through these maps:
Tunnel of Past Loves
Free of Followers Corridor
Hidden Treasury

At Hidden treasury you can get the Jade of the Dragon from the middle chest. Try not to open other chests without first checking out the chest map since Dark Mimics have potential of killing you at this point.

With the Jade of the Dragon in hand, teleport out back into the control room. Teleport to the Floating Ruin of Plaza of Bastea to walk back down the path again. This time you want to go left from Keepers’ Battle Observation Deck.

You can now unseal the gate at the Den of Dragon for the Throne of Dragon. Make your way toward the center to fight and kill the Dragon King for its Testament of the Dragon King at the heart of the Throne of Dragon.

With the Testament of the Dragon King, head back to Floating Ruin of Plaza of Bastea. Unlock the seal to continue your journey to the tower.