A possible direction I could take the Grail of Fire series in, with a stronger Fate/Grand Order tie in.

No guarantee this is actually the way future chapters go

Omake: The Grail of Fire - The Singular Order of Things

-o-o-o-o-o-o-o-o-

In the Gryffindor girls' dorm, Iris awoke with a shout. Sweat poured from her body and her breath came in ragged gasps.

She blinked slowly into the midnight gloom as her brain ran through practiced patterns for dream analysis.

It has been too literal to be portentous. Too varied in perspective to have been a memory. Too… everything to have been just a dream.

Her heart rate, which had been returning to normal, picked back up again as she threw her blankets aside and lunged from her bed towards the door of the room.

When she reached the door to the common room, she stopped. Releasing an annoyed huff, she grabbed the doorknob, closed her eyes, turned away from the door, then gave a sharp tug on the knob.

She knew she was successful when the knob warmed up slightly and began to reconfigure itself in her hand.

Not willing to wait for the knob to fully change into the statuesque lion's head as she knew it would, she gave a quick twist on the morphing handle and pulled the door open.

What lay beyond was not the Gryffindor common room, but the large office that had been taken over by the summoned version of her own older self, Caster.

The woman in question hovered over one of the many tables that were strewn throughout the room.

Said table held a screw, an egg beater, a fishing hook, a bottle of paint, and a stick of incense. Nothing else.

Iris glanced at the random objects, wondering just what her older self was planning to do. She was completely sure that they were connected to some event which would be adding more grey hair to the heads of the people running the awkwardly titled Sept-Wizard Showdown. How that would happen though… and with those specific items… the details eluded her.

She shook her head to clear the distraction and stared at her older self.

Caster looked up at her intrusion and slowly raised an eyebrow.

They stared at each other in silence for a few minutes while Iris worked her mouth, trying to come up with something to say.

"So." Her older self broke the silence. "From your expression, I'm guessing you've reached that part of the dream cycle." A wide grin spread across her face and she flickered across the room to stand beside Iris. She gently nudged the girl into the room and closed the door behind her. "I'm not exactly sure why you're surprised though. I'm a Heroic Spirit. That's not exactly a title you earn by collecting bottle caps."

"But! What about—!" Iris waved her hands around in an attempt to compensate for her inability to find words to describe what she had seen." I thought—! … Voldemort?"

Caster laughed as she directed Iris to sit in an overstuffed chair that hadn't been there a moment ago, then passed her a steaming cup of tea, which she apparently pulled from thin air. "I supposed Dumbledore might be cross with me for besmirching your adorably innocent worldview, but allow me to impart a few facts about this world." Caster took a seat on her own newly extant chair and began to sip from her own cup of tea. "Against the scale of the world, Voldemort is nothing.

"Against the collective of humanity, actions that crash through the Wizarding World barely cause a ripple, because these actions are naturally hidden from the great majority of humanity.

"Voldemort could conquer all of Magical Britain and rule in darkness for decades, and he wouldn't even be a blip on humanity's radar. And likewise, crushing him earns us mention in books and papers that will only ever be read my magicals."

Iris stared at the older version of herself, letting the memories from her dream play out again. "Wow…. So you…."

"Yup! And do you want to know the best part?"

Iris nodded excitedly.

"I got to name it!" Caster grinned.

"It?"

"Yup. I called it Ragna-rock."

Iris blinked a few times then let out a mournful groan and pressed her face into her hands. "That's so bad!"

"I know right!?" Caster cackled. "And they went along with it. That's in the history books now…. Well, it will be in the history books."

"Is that how you actually ended up on the Throne?" Iris glanced up between her fingers. "Did Hermione kill you for making such a deplorable pun?"

Caster smiled and leaned back in her chair. "Nah. Not for lack of trying on her part though. I don't suppose the dream cycle showed you the look on her face when all was said and done."

Iris shook her head.

"Shame. Maybe next time. If you don't get a dream vision of it, make sure to watch her closely when the events actually pass for—" Caster cocked her head to the side and appeared to be listening to something that Iris couldn't hear. "What the hell?"

Caster stood from her seat and walked towards a window which hadn't been there before and overlooked the Hogwarts grounds from somewhere nowhere near the Gryffindor tower.

Iris placed her teacup down on a convenient end table and went to join her senior self at the window.

They both watched as a thick fog rolled in from the forests and began to extend across the castle grounds like a reaching claw.

Iris squinted her eyes as she tried to pick out small hints of movement she spied in the soupy mist.

Caster must have seen them as well, but she was better equipped to take action.

With a sweep of her hand, a sharp wind blew through the area, pushing the fog back and revealing a small band of moving skeletons that had been hidden by the mist. Another twist of her hand caused lighting to strike all the skeletons at once, shattering them.

"Inferni?" Iris asked.

"No. Inferni are just animated corpses, with no remnants of the original being within them. These are actually lesser undead. Reanimated bones moved by hateful emotions they possessed in life…." She trailed off as they both tracked a barely perceptible shadow as it scampered across the grounds away from the castle.

Caster sent another spell after it, but it handily dodged the attack and vanished into the forest, barely disturbing the mist.

"Presence concealment…." Caster glanced up at the full moon that had allowed the two of them to see in the dark as though it were mid-day. "And it definitely wasn't X. Has someone else summoned an Assassin?"

The fog once again crept towards the castle, only to be turned away by Caster's wind. The revealed skeleton warriors were each struck with Explosion Curses this time, wiping them out.

'Awaken!' Caster's voice echoed through Iris's mind. 'Everyone awaken! The castle is under attack.'

Iris felt a shiver run down her spine as she spotted seven silhouettes standing at the edge of the fog.

Caster spotted them too. 'All Servants, prepare for combat against enemy Servants.'

-o-o-o-o-

A few days had passed and the noisy panic of the first night had settled into a much quieter panic. The Great Hall was filled with students that had come down for a breakfast consumed between hissed whispers to their fellows.

Classes were canceled, and the grounds were off limits to anyone that didn't want to be attacked by wandering undead, so the students that finished their food quickly simply went back to their dorms to wonder about what had become of the rest of the world.

The teachers could give no assurances on that either. No form of communication, magical or mundane could reach beyond the fog that surrounded the castle. The Floo wouldn't connect. Shirou had made a trip to the edge of the grounds and back before reporting that Apparition and Portkeys simply caused the user to rebound back to their initial location.

Hogsmeade could be reached through some of the castle's secret passages, but the town was completely empty except for a few wandering wraiths, leaving no sign of where the inhabitants had gone.

Durmstrang had abandoned their ship and Beauxbaton their carriage. Both sets of students huddled in the castle along with their Hogwarts counterparts.

The initial assault from the enemy Servants had been repelled, but the Goblet of Fire had been taken, and Caster was convinced that their enemy would use it to summon more Servants than they would be able to defend against.

The air of the castle was thick with tension and fear. Iris could practically feel it dragging at her limbs as she mechanically moved food from her plate to her mouth.

The Servants of the Tournament had been the only real defense the Castle had, and they were constantly active keeping the mist and the undead at bay.

It was convenient for the security of the students that the Servants didn't need food or sleep, though continuous activity from them, without the Goblet nearby to sustain them, caused a significant drain on their Summoners.

'Iris.' Caster's voice resounded in her head. 'I need you to open the front gates. I'm providing cover to a group retreating from the undead horde.'

Iris immediately dropped her spoon and jumped up from her spot at the table, rushing towards the Entrance Hall.

Once there, she pressed her hand against the giant wall of steel that Caster had put in place over the door.

At her touch, the steel peeled back from the large castle doors. The giant wooden beam that feel across the doors lifted itself away, and the doors themselves opened with an ominous creek.

Two people rushed in, followed a moment later by Caster herself, who sealed the entrance again with a wave of her hand.

Iris's eyes were naturally drawn towards one of the new arrivals. It was a young woman, and Iris was convinced that she was a Servant.

Not because Iris had any particular skill for spotting Heroic Spirits or anything, though she did have a bit more presence than a normal person. No, it was because this girl was wearing an absolutely ridiculous set of armor, but still managed to look cool, cute, and beautiful while doing it.

"Thank you for the help," the probably-Servant said once she caught her breath. "My name is Mash Kyrielight, a Demi-Servant." She gestured towards the person next to her, who was still gasping for air. "This is my Master, Ritsuka Fujimaru. We're here to resolve the distortion in reality here before it becomes a Singularity."