| I should have been the one to fill your dark soul with liiiiiiiiiiight! |
When you start, turn around and go back into the room where you fought Nightmare. There is an Untouchable hiding to the right side. From there, go back to the circular room where you started. To the right, beneath the door covered by claw bones, is a Holy Water.
If you continue running around the center, you'll find a Divinity Statue half-covered by organic stuff. Directly across from this is a secret room. Just running into it isn't enough; you have to examine the wall. This will start Secret Mission 12.
When you're ready to proceed, zone through the only unsealed door, to the right of where you found the Holy Water. This takes you to the heart chamber. The giant door is blocked, so this is a dead-end for now. There are platforms to Dante's left. Jump along them and you will find a giant red ornament-like thing. These are the underworld equivalents of the activation devices you're familiar with. Some also function like Red Orb Crystals, so it's generally a good idea to smack any that you see!
Once the ornament is broken, the bones blocking the other door in the round room will open. Head back there and through the once-blocked door.
The undulating tunnel beyond is host to a couple new environmental disasters. First, standing in place for too long will cause a parasite to pop out of the ground and bite at him. (They kind of look like prototype Faults.) Second, there are wavy colorful bloodfeeding tentacles sticking out of the walls and floors. If Dante gets too close, these will latch on and pull him in while dealing damage. Get enough of these on you, and your health bar will drop rather fast. To avoid them, equip Ebony & Ivory and blast away while walking through.
There are two paths when you first start. The right-hand one is a dead-end, so go to the left. Shoot your way through the tentacles and you'll get blocked in with a Nobody. Kill it and continue. There will be another Nobody, some more tentacles, and a third Nobody. Fight your way through all of these to get to a lava-filled room with stepping stones forming a path across.
Touching the lava will hurt you, just not as much as if a normal person were to step in it. To complicate things, there are some Plasmas here, too. Because there is so little floor space here, it is adviseable to use ranged attacks against these. Once they're dead, cross to the far side of the room and smack the round device to raise some pillars. Jump on top of the pillars to get to the next ledge. There is another ornament here, which grants Red Orbs when struck.
Activate the shield device to the right of the ornament to cause more pillars to rise. Jump across them and zone into the next area.
Dante finds himself back in the heart chamber, but on a higher level. Leap across the chasm and smack the shield device to activate the heart. The beating organ will dispel the powers blocking the final door. Jump down into the bottom part of the chamber and examine the large door to end the mission.