I actually had this idea for awhile, I just didn't know how to write it. This is during the fight in Day 6.

Magenta danced around Mikoto even more and molten concrete spread out around his feet, lava bubbling like boiling water. Reisi hoped that it was only him imaging seeing the Red Sword become even more damaged than it already is.

With all the damage Mikoto's taken he lost focus on controlling his red aura momentarily. Now that it freed itself from the man's control he could no longer put the reins back on it. It was like it had its own will to destroy everything.

"He's completely lost it..!" Reisi muttered, taking a step back and using his aura to shield from the blistering heat. "Is he going to cause a Damocles Down like Kagutsu?!"

"...Archer, you and your Master stay back. I...I'll handle this..." Lancer said quietly, almost whispering. He was biting his lower lip so hard it bled and his grip on his spear tightened even more.

Archer narrowed his eyes in concern. Though both are enemies, that seems to have been forgotten with the present situation. "Are you sure you're mentally able to kill your own Master?"

Lancer sighed and tried to force a carefree grin despite him fighting the urge to cry. "It's not like it'll be the first time I've killed a friend."

"You've killed your friend before?!"

"I'll tell you my True Name...It's Cú Chulainn."

"In that case you should allow my Master and I to handle this."

"Ya think any of your attacks will reach him? No, it has to be me."

The air several meters around Mikoto blazed with the temperature of an erupting volcano; inside that radius had to be even hotter. But Mikoto was somehow getting off easy with only his skin being charred black. Just being within under a dozen meters would burn an unprotected King away like a straw doll. A normal Servant would be severely burned standing a mere few meters away from the redhead. Though he's not as powerful as Lugh due to being a demigod, Lancer has the same protection against temperatures as extreme as from stars since he's the son of a sun god. So this unbearable heat wasn't affecting his body at all as he walked toward his Master. You could say this situation makes him the perfect partner for the Red King. As someone who's capable of standing against gods, this task is no problem for Lancer even though he's at only a fraction of his full power.

"So...this is you at full power," he murmured as he got close enough to Mikoto to almost touch him. "I now see why ya tried to keep it tucked away..." He then put his left arm - the one not holding his spear - around the King, pulling his body in close. A space of calm serenity, unaware of the fire billowing around.

"Lan...cer..?" Mikoto said almost in a whisper then his eyes briefly widened, blood spilling past his lips. The bloody head of the demonic spear was now outside his back, through his heart.

A muted grunt from the spearman as he felt his spirit core shatter. It mattered little to him. He got to fight strong opponents so he's satisfied with dying when his Master does.

"Sorry Mikoto," Lancer whispered, "but I had to do this." The grip of his right hand on his spear didn't waver.

Warm arms wrapped around Lancer. "Thank you..." Mikoto breathed before all his weight slumped on his Servant, soon wrapped in a flurry of blue mana as his Sword of Damocles dissolved in a mist of red.

Dead End