You will find the Concerned Monk just inside the entrance to the Planetarium after speaking with Antanas at the top of the Citadel for the first time. He’ll ask you to go and locate his brother who has gone missing.
Return to the Initiation Room (where we fought the first boss) and go through the left hand door (where we went after killing the Warden). There is a Shieldman and a Rogue waiting for you. After killing them, proceed down the first set of stairs and enter one of the doors ahead. In the small room between the two adjacent bedrooms, there is a weak wall . Sprint at the wall with your shield raised to go through.
Pick up the Cellar Quest Key from the ground in front of you. As you enter the next door, a Spider will attack you from the right. Try to fight him around your current position as there is an Infected a little further into the room as well. Once the spider is dead, ignore the two doorways below and instead move to the far end of the room and kill the Infected. Grab the Human Skull item nearby. Continuing past his location, there will be 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 located, run up and charge this with your shield up to break it down. Inside you’ll find an Empty Bottle .
Exit the hidden room and note the nearby doorway with the lever next to it (we’ll exit through here when we’re done!).
Once you have investigated the cocoon, you can return to the Concerned Monk in the Planetarium to give him the good news for 4,500XP .
You should now also be able to unlock the previously locked chests either side of the Concerend Monk for some new Armour (Chest, Legs and Wrist - Persistance) .