The Portal
After activating the final shrine, something will have triggered and the portal will be open.
Go to the back area of Radiance to find a path leading downstairs.
All the leaders will be there before the portal. However, only you can enter the portal. This means your party will have to stay behind.
As you emerge on the other side, you will see that the Darkbringer jumped into the portal as it closed. You listen to his argument and agree to let him tag along with you for now.
Head to the center of the map to speak with the ember Goddess.
The ember Goddess says that she is bound by these seals so you offer to break them for her.