| I should have been the one to fill your dark soul with liiiiiiiiiiight! |
If you didn't grab the Blue Orb Fragment from the side room in Mission 18, now's your last chance. Jump onto the second floor walkway on the left-hand side of the room and enter the doors there. Follow the red orbs down to the end of the hall and pick up the fragment.
When you're ready to continue, step on the red and black portal left by Nightmare to return to the guard's room. Exit the room and run down the spiral staircase. Halfway down is a room with a Marionette in it that you can kill if you'd like, or you can bypass it. At the bottom is another small room with yet another red and black portal. Step on it to be taken back to the Broken Bridge.
The Elixir will react to the image of the castellan's bedroom. Jump through. If you didn't visit this area in Mission 16, there are some goodies around the bedroom for you. On top of the bed is an Untouchable and beside it is a Blue Orb Fragment.
Both of the doors in the room have disappeared, so the only way you can go is through the mirror. Before you exit the room go back to the bed to find Nightmare-beta. Exit the mirror world bedroom into the courtyard. There is a Blue Orb Fragment hovering just off of the turret. To get it, climb to the highest level of the courtyard, jump, and use Air Raid, then fly over to it. If you don't have enough DT or miss it the first time, you will shortly be involved in a fight, and can recharge it then. Alternatively you can simply jump off the turret to get the fragment. Also, if you pay attention, you will notice another fragment spawning on the tower to the left. An Air Hike from the top of the turret will be enough to get there.
On the bottom floor of the courtyard on a pedestal is the Philosopher's Stone. Grab it to be introduced to the Nobody, a creepy, skinless, ape-like demon that can grow in size with the help of a magic mask. Easy ways to deal with them are Air Raid or Round Trip. They're fast and powerful, so stay on the move. Once they're all dead, the door will unlock. Climb to the highest level again and jump down through the hole in the floor to get back to the door.
Exit through the mirror. In the bedroom again, step on the blue portal in the floor to return to the Broken Bridge. Jump across the bridge pieces to go back to the cathedral. Here, the pool of blackness in the floor will react to the Elixir. Go stand in and examine it to be dropped into the mirror world cathedral. Cross the swirling clouds in the floor and examine the yellow decoration in the floor to use the Philosopher's Stone. The clouds will part, leaving a broken glass dome behind. Jump through it to end the mission.