Mordred Alter let his Instinct guide him as he swung at Diarmuid every time he tried to move past the Pretender, the Berserker always jumping back and returning to leaping about the room to try and disorient his opponent. Diarmuid continually attempted to get around Mordred so he could get to his target, King Arthur, the King Arthur he faced and was betrayed by in his eyes. Thankfully the narrow passage the fight took place in made it simple for Mordred to block the path as if he were one of the Spartans at the Hot Gates. The Berserker's greatest skill was his maneuverability and having such a small space made it hard to utilize that as an advantage. Mordred found his own greatest asset, Avalon, less useful than normal given Diarmuid wasn't trying to kill Mordred, he just wanted to get past him.
The greater speed and the brief accelerations his Mana Bursts created made Diarmuid far faster than Mordred, but the Instinct of the false King Arthur allowed him to avoid losing track of the Berserker. It was also helpful that Mordred could extend his reach a good bit with his own Mana Bursts. The Pretender could destroy the whole area of the tunnel they were in using his lightning to try and tag Diarmuid if he wanted, though it still wasn't a guaranteed hit and doing that could remind the Berserker of just how fragile their surroundings were, at least by the standards of beings like them. That fragility was something that could be taken advantage of to get around the false King Arthur. Servants were strong enough that they could just run through solid stone as if it weren't there, though at the cost of it being difficult to determine one's surroundings.
It was unfortunate for Mordred that the Berserker managed to come to that realization without inspiration. Diamuid looked like he was heading Mordred's way again, but then he turned towards a wall and dove into it. Mordred used his Instinct to detect where in the walls Diarmuid was. He had already gotten past the Pretender in the millisecond he had been swimming in the stone. A slash infused with gold and black lightning ripped open the wall and the entire side of the cavern as far as Mordred could see. Diarmuid had dodged and was now underground, but the detour forced him to slow down. Mordred dashed over to where the Berserker was, the false King Arthur thrusting his arm into the ground and grabbing Diarmuid. Armored fingers pierced into the Berserker's shoulder as Mordred pulled him out of the ground. Not letting go, Mordred stabbed Excalibur at Diarmuid who blocked with his red blade. Diarmuid tried to retaliate by cutting the Pretender's arm off, but Mordred lifted and swung the Berserker through the air. The slash missed as Mordred slammed Diarmuid into the ground before trying to stab him again. He had to divert that swing when Diarmuid thrust his spear. The weapons banged into each other and released a clang that echoed throughout the cave. Both armaments and the arms that held them bounced backwards, but the Berserker still had his sword. Diarmuid's blade arced towards Mordred's neck. The Pretender had two options: let go of Diarmuid to dodge, or use Avalon which would make Mordred let go anyway since he would be within a different dimension than the Berserker. Both choices produced the same result.
Avalon's light protected Mordred from the red blade's edge and caused his hand to lose its hold on the Berserker. Mordred immediately deactivated his scabbard and tried to grab Diarmuid, but the madman raised his legs and kicked off of the Pretender's chest. The hit was infused with a Mana Burst that sent Mordred into the ceiling while also pushing Diarmuid underground. The armor on Mordred's chest crumbled, as did his ribs which stabbed into his lungs. The Pretender's chest caved inward and Avalon went to work healing it.
"Saber!" Diarmuid popped out of the ground and jumped down the tunnel towards his quarry.
"Get back here, you shit!" Mordred slipped out of his restrained persona as he threw Excalibur at the Berserker. Diarmuid dodged around the blade, but Mordred was already running at the raging warrior, leaping onto him. They began to wrestle on the ground, Mordred trying to restrain the madman.
"Saber!" Diarmuid elbowed Mordred in the face, denting his helmet and breaking his nose. Another elbow shattered several of the Pretender's teeth. Mordred hit the back of his fist into the Berserker's liver which caused him to vomit. They continued rolling around and hitting each other whenever they found an opening, Diarmuid trying to get free and run while Mordred did his best to keep the Berserker in place. While Mordred's wounds healed, Diarmuid's piled up. The madman could regenerate to a degree since he was a Servant, but it was far more limited, even with the Grail giving him magical energy. He did have the benefit of holding both his Noble Phantasms while Mordred didn't, but using them in a grappling match wasn't simple.
"I'm not gonna let you hurt my mom!" Mordred was too focused on wrestling with Diarmuid to keep up his usual demeanor that imitated his ideal King Arthur. "She did nothing wrong!"
"Nothing wrong? She betrayed me!" The Berserker nearly squirmed his way to freedom, but Mordred jumped back onto him.
"She didn't know about her Master's plan!" Mordred had heard a little bit about the Fourth War from her mom in preparation for the Fifth. Mom wasn't coordinating with Kiritsugu Emiya at all so she didn't deserve any blame over what happened to Diarmuid.
"She knew what kind of man her Master was! She decided to serve a man without honor! She enabled him to shame me!" Diarmuid thrust the butt of his spear into Mordred's collar bone, breaking it in two.
"It isn't her job to watch her Master like he's some little kid, and he was just doing what he had to! It was war!" Mordred grabbed Diarmuid's arm.
"She's a villain who ruins everyone's lives!"
Mordred was originally going to retort, but then he remembered both the lives he lived in Camelot, the history where he was killed by King Arthur and the one where he killed King Arthur. His mom's actions had left Mordred isolated and miserable, but that was okay because Mordred deserved it. He was the monster who caused Camelot to collapse because of his childish spite. But she also caused Lancelot's betrayal, though she only caused it because she was too nice. She was just too good for the rotten people in this world. But it wasn't kindness that made her reject Mordred, it was fear and suspicion, she said so herself. Mom was being honest when she said that. She admitted she was wrong.
Mordred was wrong too though. They were both wrong. They were both culpable in the fall of Camelot, but Mordred was far more responsible as the one who incited the insurrection that tore the kingdom down. Mom was still partly responsible though. Mom made mistakes. Mom wasn't perfect. Mordred knew this, he kept remembering this, and he kept remembering he shouldn't hate himself. Despite thinking that over and over, Mordred kept on falling back into self-loathing. Why? Old habits? He had spent decades hating himself and worshiping his memory of King Arthur, the habits such a life created couldn't be shed in just a few weeks.
Was there any point in trying to change in that case? If this cycle of hollow progression followed by immediate regression was going to happen every time, perhaps it would be better if Mordred gave up and just remained as he was.
Soon enough Mordred got on top of Diarmuid's back and caught his legs under his armpit, Mordred using his free hand to grab the Berserker's head and pull it back. The only things the mad warrior could use were his arms and the weapons they held, but the angle made hitting anything near impossible.
But it wasn't impossible as the tip of the red spear headed for Mordred's back. If the Pretender used Avalon, then Diarmuid would be completely freed, so instead Mordred opted to release the head in his hand and reach back to grab the spear just past the blade. This would have been the perfect opportunity to break the spear and Mordred's biggest headache with it, but the angle his arm was at made it impossible to gather the necessary strength to do so even with a Mana Burst. Instead the Pretender prepared to release lighting in all directions from his body to destroy the Berserker while he couldn't run.
"Moralltach!" Diarmuid calling his sword's True Name caused his legs to gain a boost in jumping power and thus became strong enough to wrench themselves free from Mordred's arm, breaking it. Mordred was knocked away as Diarmuid leaped high, his sword and spear both raised. They glistered as they crossed before the Berserker shot downwards and swung both instruments, releasing magical energy that formed into three great blades full of Divinity.
"Avalon!" Just as Diarmuid descended past Mordred, the Pretender activated his scabbard. The triple blade of energy failed to cleave the false King Arthur into four pieces. Mordred touched the ground while the Berserker was already running after mom.
The Pretender scooped up the dual-colored Excalibur as he ran after Diarmuid. The Berserker's jumps made it impossible to keep up with him and Mordred wouldn't trick the madman by throwing Excalibur like last time. Diarmuid wasn't so insane that he couldn't strategize and remember past deceptions.
Mordred stopped running as his arm finished regenerating. The false King Arthur's helmet disassembled as he held out his half-corrupted Excalibur with two hands. The blade erupted with golden light and blazing darkness that shot upward into and through the ceiling. The shaft of twin energies broke out of the surface of the mountain before swirling together into a tornado of black and gold. Mordred Alter could not use the proper True Name release of Excalibur due to its partial impurity, but he could still use a modified version of the Holy Sword's true power.
Diarmuid kept on running towards mom. She was all he cared about. King Arthur had become the icon of all he hated. He was the opposite of Mordred Alter in that sense, and yet they were so similar. They were both obsessed with the image of King Arthur that existed in their heads. It blinded them. It wasn't healthy. Mordred had wondered earlier if there was any point in trying to change, but to give up trying was to become like this corrupted Diarmuid, a single-minded monster who would let the entire world die for the sake of his vengeance.
If Mordred was ever to save himself from such a fate, he had to grow or at least try to. His self-loathing and idealization of his mother had to be overcome. Mordred had to forgive himself. He had to accept that his mom was just a person like anyone else. No more exaggerated perceptions. Mordred had to look at reality for what it was, something complicated. Pretending it was full of clear cut rights and wrongs was only going to cause Mordred to drown in the nuances. It was time to move forward.
"Excalibur-" Mordred took a step forward and raised his sword high.
"Saber!" Diarmuid's voice echoed.
"Camlann!" Mordred swung his arms down and a spiral of darkness and light rampaged down the tunnel, demolishing the edges of it and far beyond. Diarmuid was destroyed before he even had a chance to think about dodging. The blast erupted into a tower that pierced out of the mountain and devastated even more of the cave system.
The dust settled and what was in front of Mordred now was a massive cavity in the mountain with no ceiling.
"Holy Hell."
Mordred turned around to see Rin who had an unconscious Illya on her back. The magus in red was missing an arm, a cauterized elbow being all that remained of it. Rin's steps were all unsteady, like she would collapse at any moment.
"Are you two okay? Where's Assassin?" Mordred ran up to them and took Illya off Rin's back to ease her burden.
"Assassin's dead, but so is Gilgamesh." Rin's face twitched for a moment when she spoke of Assassin's demise. "I'm exhausted and Illya's out cold. What happened after we left to fight goldie?"
"I ended up having to deal with Diarmuid, which I just finished doing." Mordred pointed his thumb at the missing chunk of the mountain. "The others should be in the Grail chamber by now, but it's hard to tell with all the cursed magical energy wafting through this place. It's hindering my senses."
"Same here. It's good that you dealt with Diarmuid. Are you gonna help deal with the Grail now?"
"Yes. I was lucky enough to have avoided any major injuries thanks to Avalon. I used a lot of magical energy just now, but the Lion King provides me with so much, even in her weakened state, that I can still keep going."
"That's good. I can't and obviously neither can Illya. Even if we healed all our wounds, we used too much energy and that can't be restored with Avalon or anything like that.'
'You should teleport to safety then."
"Already planning on it." Rin pulled out a green rune that hovered at the tip of her index finger.
"Wait." Illya started to wriggle around in Mordred's arms, consciousness having returned to her. "I'm staying."
"Illya, you're in worse shape than me, well, if you ignore the arm thing." Rin said. "You'll only get in the way here."
"I need to see mama. I'm not leaving until I save her." Illya's eyes opened and there was a heat in those rubies that reminded Mordred of the fire he saw in her mom's eyes when she made a similar declaration. Mom was a mess after defeating Lancelot, and yet she still insisted on staying to save mama-er-Iri-Irisviel. "I'm going to mama, even if I have to get through both of you."
Rin sighed and pressed her palm into her face.
Mordred reached into himself and pulled out his Avalon.
"What are you doing?" Rin tried to take a step, only to nearly fall over.
"With two Avalons, Illya should heal enough to move on her own." Mordred guided the sheath into Illya's body, the scabbard joining its counterpart inside the homunculus and speeding up her regeneration.
"You're seriously going to let Illya stay?" Rin said.
"She needs to be there for Irisviel. Having as many important people in her life present will increase the chances that we can reach her."
"This is a terrible idea, but I guess I can relate to the sentiment." Rin's body relaxed. "Fine, but don't get so obsessed about saving Irisviel that you all let yourselves get killed."
"We won't." Illya got to her feet. "Our whole family is coming out of this okay. That includes me." The homunculus looked at Mordred. "That means you too by the way."
"W-Why?" Mordred leaned back.
"What d'you mean, why? You're a part of the family." Illya put a hand on her hip.
"But I'm not your Mordred-"
"You have his memories?"
"Yes."
"You care about our family?"
"Well, y-yes. I don't want you people to die."
"That's good enough for me. Let's get going." Illya ran deeper into the cave without another word, her body a blur.
Mordred was both stunned by Illya's acceptance of him and the fact that she somehow was able to move so fast.
"Here." Rin took off the winged sandals she was wearing, holding them out to Mordred. "They give you a speed boost. I projected them during the fight with goldie."
"Thank you." Mordred took the footwear and replaced his boots with them. "It feels strange to wear my armor with opened toed shoes. Anyway, I'm going after Illya. Be safe."
"Be safe."
Mordred took off with greater speed than he had ever previously achieved. While he caught up to Illya, he thought about how easily the homunculus accepted him. The Pretender was struggling to not hate himself with all his being, and yet the daughter of the woman Mordred had wronged more than anyone was willing to accept him as a part of her family without a second thought. Illya thought that the very idea of Mordred not being part of her family was ridiculous. It made Mordred feel guilty for some reason. That said, Mordred supposed that if he was ever going to forgive himself, doing it while among people who had already forgiven him would be the best way.