Throughout Chained Echoes Glenn will see odd visions of a white door, and a voice will nag him to stay out of trouble. It's not until much later in the game that this phenomenon is finally addressed by anyone else, and you can learn more about it in this side quest, Into the Maelstrom.
To unlock Into the Maelstrom you first need to complete the main quest Searching for a Princess. Once you've done so - and recruited Amalia into your party in the process - you'll unlock two new side quests, one of which is Into the Maelstrom. Head to Tormund and check the bulletin board to the south of the Adventurer's Guild to begin the quest.
The notice on the board will point you to Farnsport. Jump there and head to the Docks, south of the Bazaar. On the east side of the Docks is a ramshackle section of town that serves as home to the city's less fortunate. Follow the wooden walkways until you come to a large house.
Inside you'll meet Magnolia, a depressed Soulseer who perks up immediately when she realizes that Glenn really can see a door in his mind. After some introduction to the nature of souls and rebirth Magnolia will introduce the group to a man... and, essentially, shove everyone into his mind. You'll need to guide the party to the source of his trauma to escape.
Wounded Mind
Head north, take a right, and go back south. There's a chest here containing an Elastic Wing. To the east you'll find a 'fragment' labelled House, and you'll need to remove it, also removing the path back west. To move on, interact with the next orb to the east and place House into the orb. This will create a pathway further east.
There are two more orbs on the other side, one containing House, the other containing Bridge. Remove both, then insert Bridge into the orb on the left. This will allow you to use the southern path to cross the bridge. On the other side you'll find a chest containing three Fine Wools.
Head back to the orbs and reclaim Bridge, then insert House into the orb on the right. This will allow you to proceed east and north. Go all the way north to find a chest containing two Hard Shells before you do anything else. That done, check the orb in the east for the Cliff fragment, though leave it in place for the moment.
Climb down the handholds on Cliff and head east. You'll find a chest containing three Platinum Ores, as well as an orb in the far east. Insert Bridge into the orb, then return to Cliff. Grab the Cliff fragment from the eastern orb, then install it into the western orb. This will allow you to climb down south and grab House again. Put Cliff back where it was in the east, then place House into the final orb to the east of Cliff.
If done successfully this will join House and Bridge, and you'll be able to access the final area in the northeast, where you'll learn why the man's in a coma. Harsh. Check the glowing yellow scar to the left of the man to exit the Wounded Mind. The 'Radiant Diva of the Equilibrium' will thank you for your help, explain a bit more about the door in Glenn's mind - though not enough - and give the group a Rusty Sword as payment. It's a better prize than you might think, though you can't do anything with it for a little while. Quest complete.