After acquiring the Wheel of Destiny from the underground chamber, the trio returned the way they'd originally came back to where the bridge had raised behind them, locking them out of the castle. This caused the bridge to lower for them.
"So...all that just to pick up that Staff of Hermes?" Applejack asked, referring to the one key they'd picked up that hadn't been used yet.
"Guess we'll figure out what it's for inside," Dante suggested.
Back inside the castle, they found all the lights out, neither sun nor moon giving illumination. Thankfully, Virgil had picked up a Luminite, a demonic stone that gave off light in the human world, back by the coliseum. It allowed them all to see where they were going. Activating a switch in the back of the main room caused a great deal of rumbling. Lightning struck through the skylight Phantom had broken when he got impaled and struck the statue Phantom had been impaled on. A bat made of lightning became visible.
"One a yours?" Applejack asked.
"No," Nevan replied. "That's a Plasma. It-"
The creature was smashed between one fist of fire and one hoof of light.
"Is a corpse," Virgil corrected.
"...quite," Alastor allowed.
Continuing to explore the darkened castle, the trio came across many more Plasmas, which shapeshifted into various forms to try and combat the trio. One even transformed into a pony shape like Applejack...only to burst apart from the force of Nevan's squeals over how adorable it was. Marionettes also challenged them at various points, but proved ineffective, as before.
Eventually, they reached the chamber where they'd first fought Phantom. There were several puddles scattered around. As they approached, a voice echoed in their heads.
One who knocks on the gate to the Underworld shall borrow strength from the Blue Stone.
A massive black ooze creature leapt forth from the puddle, human bones stuck in the ooze. It let out an unearthly howl.
"It's the Nightmare!" Alastor cried as several glowing circles on the floor began sending energy into the beast. "If you're able to damage its core, you can destroy it. However, the only way to make it expose the core is to activate one of the switches around the room to change the energy it's being fed."
"I'll handle the switch!" Applejack called out. "You two stay close and attack the core when it's exposed!"
The strategy worked quite well. When the core first came out, Dante and Virgil activated their demonic forms, dealing massive damage to the core before it withdrew. They then switched, activating the switch and letting Applejack finish off the core with her Devil Trigger.
With the core shattered, the beast dissolved and vanished, and the group continued on their way.
In the path of the Trial of the Lion, they found a glowing point, reacting to the surge of demonic power.
As Dante used the Staff of Hermes, Applejack spoke up. "So...is the demonic aura strong enough for you guys to fully manifest?" she asked.
"Yes," Nevan replied. "It is quite strong."
"We shall fight again!" Agni proclaimed. "To arms!"
"Not to mention legs!" Rudra added.
"I think I picked the wrong Master," Alastor muttered. "I'm stuck with the crazies."
"Balls!" Cerberus barked happily.
"...maybe I should have waited for Virgil..."
"I don't need you," Virgil countered.
"Harsh," Ifrit pointed out.
"But accurate," Griffon pointed out.
"So is everyone else ignoring the portals I just opened up?" Dante pointed out.
"Yup, pretty much," Applejack replied.
Groaning, Dante leapt through one of the portals, followed by the others.
Following the path opened by a sequence of portals, the trio found themselves confronted by lizardmen that seemed to be made of ice. As they leapt down to attack, Cerberus briefly manifested and ate them before withdrawing back into Applejack's tail. "Yummy!" Cerberus said, smacking his lips.
Applejack looked sick to her stomach. "...you realize I just tasted that, too...right?" Nevan and Beowulf were also gagging.
"Sorry," Cerberus whimpered.
"And I'm suddenly very glad he isn't my Devil Arm," Dante whispered to Virgil.
The trio continued onward, fighting through Marionettes and Fetishes mostly while inside the castle. At one point, they encountered a giant dinosaur fossil that spit fireballs at them. Using their various long weapons of choice - Dante used Alastor, Applejack used Nevan, and Virgil using his katana - the three took turns knocking the fireballs back until the skeleton collapsed. Applejack frowned.
"What's wrong, AJ?" Dante asked.
"Remember when I toldja about hearing that almost familiar voice complaining about stairs?" she asked.
"Yes?" Virgil confirmed.
"Well, now I coulda sworn I heard that same voice shouting 'Furry Curry, Curry Cola' or somethin' like that, every time I used Nevan to whack a fireball," Applejack explained.
Dante and Virgil looked at each other in concern. "Dante, I hope your hypothesis about her hearing someone in another dimension is accurate," Virgil said at last. "Because otherwise, Applejack might be seriously sick in the head."
"I know what you mean," Dante replied. "Furry curry is gross. I should know." At the looks he got, he added, "What? I was short on cash and it was still leftovers!"
"Please tell me you never fed Applejack furry curry," Virgil asked worriedly.
"He didn't," Nevan confirmed.
Defeating the dinosaur skeleton revealed a light emitting mirror. Pulling a nearby crank caused it to rotate until it reflected off several mirrors and concentrated into an orb. Leaping up, Virgil acquired the Quicksilver.
As they retraced their steps, Applejack spoke up. "Actually, Curry Cola sounds kinda tasty. Like a spicy soda pop?"
After a time of head scratching, everyone agreed that it did in fact, sound rather tasty. "I'm going to buy a soda stream so we can experiment with making some," Dante promised.
Following another path, Virgil used the Quicksilver to open a sealed door. Behind the door was a large, cathedral like chamber. "This seems like another good place to spar," Virgil muttered.
"Spar back home!" Applejack scolded. "We've got a demon king to rip ta pieces!"
Chuckling, Dante turned to lead the trio on their way.