Undyne stared down her opponent, garbed in armor as she was, as the yellow soul floated around her, giving off its crimson glow. She bared her teeth as she stared into the face of her oppressors, the ones who had crafted the barrier and trapped monsters beneath the earth for so long. She rallied herself to face a foe that outstripped her in every regard...and felt her soul surge as she felt the drums of battle beating.
Gerson had once told her that every living being had a purpose. Every plant, every animal, every monster, every human, every rock, and every ghost... Undyne supposed every pony had a purpose to. As she felt her blood boil in her veins and the yellow soul resonate with her own, she realized...she'd found hers. The impossible struggle, the battle for the fate of her race...this is what she'd been born for, and she would not fail!
"Heh," she grunted as the yellow soul swept closer and closer to her in the ether. "You're just as eager for this as I am. We're outmatched by an unreasonable margin...but despite that, you're as ready as I am to leap into the fray. You'd attack this guy even without me, wouldn't you?" Reaching out, she let the soul settle into her hand. "Well...I can hardly keep you waiting, now can I?"
She stared up at the armored Sage, wielding its twin swords. "You think you just have a monster with a human soul to deal with here, and are convinced you can handle me...but you're wrong on both counts. First off...I'm not alone. Every monster in the Underground is behind me, and more. Can you hear it?"
The Sage tilted its head in confusion.
"The voices from beyond the void..." Undyne whispered. "They want Woona to be saved...they want us to be free...their power flows into me as well!" She clenched her hand around the yellow soul, and her body flared with light. "They all stand with me, united in a desire to bring you down!"
Undyne's armor blazed with light as it warped around her. The grey metal darkened to black as it shaped itself around her, customizing itself to her as full plate. The shoulder epaulets expanded, becoming three-fold spikes to either side of her head. Her crimson hair burst free of its ties, flaring behind her head like a banner. Her muscles bulked up within the armor as power flowed through her body, and marks of hearts appeared on her gauntlets and grieves. A yellow heart appeared on the armor in the center of her torso, pulsing with light before being encased in crimson crystal. Her eyepatch burst into light, and the empty eye socket opened, revealing the yellow soul staring out with her. She seized a spear of magic out of the ether, and it crystallized into physical matter, a flaring blade of ruby and steel.
"And your second mistake," Undyne spoke up, "was thinking you could ever handle me!" Thrusting her arm forward, a wave of blue magic spears lashed out at the Sage, some being deflected off its swords, some bouncing off its armor...but some hit their mark.
The Sage lunged forward with its twin swords, seeking to counterattack, but Undyne deflected each strike with a spin of her spear. She watched and waited carefully for an opening...and when she saw it, she clenched her free hand and delivered a haymaker right to the Sage's chin, sending it flying back. Lunging forward, she began rapidly thrusting forward two handed with her solid spear, forcing the Sage on the defensive as she shouted out, "Ora ora ora ora ora!"
The Sage continued to use swords and armor to deflect most of the blows, but the pace was wearing on the ethereal foe. More and more strikes were penetrating defenses, and it was forced to leap back out of range. It tried to get enough distance for breathing space, only for a sweep of Undyne's spear to stretch over it, trapping it in a green glow, unable to move.
"You should know what green means without me telling you," Undyne growled out as she shifted her spear from her right hand to her left. "After all...you've kept us from moving on for eons!" She lifted her right arm...and was stunned to see a massive mini-gun take shape out of the ether, courtesy of the yellow soul. "Heh, kid I like your style!" She aimed the massive weapon at the Sage and opened fire, shouting, "Daka daka daka daka daka daka daka daka!"
The mini-gun fired out not bullets, but her magic spears, barraging the Sage from all sides as it was forced to spin and twist, desperately trying to deflect all the energy spears that sought its life. Deciding to add to its struggle, Undyne added a rain of spears from above, striking down at it firmly before hurling her main spear as the green curse lifted. Even the shockwave sent the Sage tumbling.
Undyne watched her foe slowly but surely becoming her victim, unable to keep up with the sheer amount of power she was able to put out...and knowing that even at this point, she was still holding back. Is this what it's like for the skeletons? she thought to herself. All this power inside, knowing how easy it would be to just turn it loose to handle anything they came across...knowing it's only their own will that keeps them from changing from monsters into demons? She chuckled softly to herself as she closed her eyes. Maybe I've been too hard on Papyrus...if this is the kind of power he keeps in check, maybe he'd make a better royal guard than I thought.
Seeing her eyes closed, the Sage leapt free of the barrage, taking several strikes as it raced for her to deliver a heavy blow. With its killing intent and power, a single blow would be all it would take to slay a Monster, even strengthened by a human soul. Leaping into the air, it brought its swords down...only to feel them fall from a suddenly nerveless grasps as it stared at the spear it had impaled itself on.
"And your final mistake...thinking you could read me," Undyne finished as she opened her eyes. "This fight is over...and now you die!" Jabbing the base of her spear into the ground beneath her feet, she leapt into the air. "This is for every monster you ever killed in the Great War, you bastard!" With that, she came down boots first, driving the Sage down onto her spear until the blade punched through its soul, shattering it.
Undyne landed beside her spear, pulling it from the ground with a spinning flourish. "That's mine down, Woona!" she called out as she turned towards where she could sense the pony's struggle. "And if I won here...then you don't get to die!" She laughed maniacally to show her confidence in Woona, to give her the encouragement to hold on just a little longer.