Radiance Passage
After entering the trapdoor, you will be in a hidden passage to Radiance. Coren speaks about this place for a brief moment.
This is a simple hallway that will have gates. When you approach these gates, a guardian will activate. Simply defeat these guardians to get through the gate they are guarding.
The last guardian is the toughest (in a tanky way). It also summons a few allies to attack your party. Deal with them the same way you dealt with multiple enemies before. The allies it summons aren't particularly strong.
Unlike all the other guardians, the White Guardian drops a special weapon called Greatsword of the White Guardian. It has excellent stats and also comes with the Lightning Strike skill.
When you exit the passage, you will be in Radiance. The place where Coren lives in. You may ask him about this place if you want more lore.