The Concerned Monk

Speak with Antanas at the top of the Citadel for the first time. The Concerned Monk will then be found just inside the entrance to the Planetarium. He will ask you to locate his missing brother.

Return to the Initiation Room (where the first boss was fought) and go through the left-hand door.

Defeat the Shieldman and Rogue waiting for you.

Proceed down the first set of stairs and enter one of the doors ahead. In the small room between two adjacent bedrooms, you will find a weak wall. Sprint at the wall with your shield raised to break it.

Pick up the Cellar Quest Key from the ground in front of you.

As you enter the next door, a Spider will attack from the right. Fight it near your current position, as an Infected is further into the room.

After defeating the spider, ignore the two doorways below. Move to the far end of the room and kill the Infected.

Grab the Human Skull item nearby.

Continuing past the Infected's location, you will find another Infected and an Audio Note (The Plot: Point Three) on the left.

Keep moving down the stairs and kill the Spider in the back left corner.

Loot the Human Skull here. Note the weak wall behind where the Spider was. Charge this wall with your shield up to break it down. Inside, you will find an Empty Bottle.

Exit the hidden room. Note the nearby doorway with the lever next to it.

Roll under the half-open gate to find a pressure plate. Enter the door it opens.

Kill the Spider inside and interact with the glowing cocoon for a quest item.

Once you have investigated the cocoon, return to the Concerned Monk in the Planetarium.

Give him the good news to receive 4,500XP.

You should now be able to unlock the previously locked chests on either side of the Concerned Monk, granting you new Armour (Chest, Legs, and Wrist - Persistance).