As soon as Toriel found herself face to face with the Sage Soul, she struck, launching a wave of her fire at her opponent. The Sage immediately countered with a wave of ice shards, freezing each fireball in the air as it approached and barraging the Boss Monster with the extra shards. Toriel rolled to the side to evade as she got a better view of her foe. The green figure was garbed in a flowing robe, and held a staff in its hands, plainly a true mage. This was going to be difficult.
Knowing that mages tended to be more skilled at range than up close, Toriel lunged in, surrounding her hands in her fire as she struck out. Much to her frustration, the Sage dodged each swipe of blazing paw, letting them sweep by millimeters from them, before countering with a solid strike to Toriel's midsection with the staff. Since the staff was charged with lightning, this blasted Toriel a ways back, as well as draining a good chunk of her health.
Grunting, she pushed her way to her feet, only to find the Sage staring down at her, not pressing its advantage.
The Sage says you have no chance, and encourages you to withdraw while you still live.
Toriel snarled as she was confronted with that detail. "You think I'll just stand by and let my child die?" she roared out angrily, leaping to her feet and firing off a massive blast of fire, only for it to be dispersed by a massive icicle that nearly impaled her, blasting by her head and grazing her ear.
The Sage reminds you that you have already done just that...seven times.
Toriel felt the tears stinging her eyes. "I know that..." she murmured under her breath. "Asriel and Chara...if I'd been more watchful, I could have realized what they'd planned and prevented it. And each of my children who fell down since...I let them leave. Their deaths are as much my fault as Asgore's. I knew what they'd face in the Underground...I knew letting them leave was a death sentence...but I could not keep them caged..." She stared at the Sage with a flinty gaze. "But no more! I have taken no action for too long! I will not stand by and let another of my children die if I can prevent it!" She once more gathered her power. "I will protect my little Woona if it costs me my life!" She felt her DETERMINATION rising.
You and me both, Mom.
Toriel gasped as she finally recognized the voice that had been whispering in her ear. "C...Chara?" she gasped out. "Is it...really you?"
It is, Mom, Chara responded. Woona's been carrying me around with her since just before she threw you that birthday party where she got you spider cider mixed with sleeping potion so she could get out of the Ruins without fighting you.
Toriel shook her head, chuckling ruefully. "That clever little foal..." Her eyes widened as she felt power flooding into her. "Chara?"
You're not fighting alone here Mom, Chara spoke up. I have a certain amount of power borrowed from Woona. Enough to match the six Souls helping the others. And I'm going to use it to help you fight this Sage, and save Woona. He continued to pour all the power he'd gathered into Toriel. It's time to show this Demon what a Monster is really capable of!
Toriel arched her back as she felt the power flood into her, filling every cell in her body with raw, unfettered magical energy. Her flames erupted from her body, her flesh no longer able to contain them as they burned away her robes before wreathing her entire body. She felt the aches and pains of eons slip away as her physical form became a full decade younger, restoring her to the full prime of her magic, the point of balance between youthful energy and grown wisdom. She hovered in front of the sage, garbed only in her inner fire as it formed wings of flame behind her, like a phoenix rising from the ashes of defeat.
Chara had only one way to describe the transformation his mother had undergone.
She's a Killer Queen.
The Sage took a step back in surprise, but steeled itself. Raising its staff, it unleashed a barrage of ice shards again, but Toriel simply raised a wall of flame to defend, making the shards melt and evaporate. She then dropped the wall before launching several spheres of flame, condensed to the point they were solid and bounced around the area seemingly at random until they came to a halt around the Sage, at which point they exploded like dynamite, unleashing a blazing torrent of flame that the Sage was only barely able to escape by encasing itself in ice. Before it could break free again, however, Toriel had focused her magic between her hands and unleashed a focused beam of flames so intense it was practically a laser, punching through the ice and smashing the Sage backwards.
As the blast ended, the Sage struggled to recover, only to get kicked in the back of the head, knocked across the featureless field as its staff fell from its grasp, bouncing away. It turned to retrieve it, only to see it in Toriel's grasp, burning away into ashes.
"You...are hurting...my child..." Toriel growled out, her eyes blazing white. "I will not allow that to continue!" She blurred forward in a blaze of light.
The Sage lifted its arms to try and protect its head, only to find Toriel's flaming hand embedded in its torso, wrapped around its soul. It stared into the eyes of the Merciful Queen...and found none.
"My child is insistent I say this..." Toriel murmured as she glared into the Sage's eyeless face. "Heat. End." With that, she squeezed her hand.
The Sage's body burst into flames as its soul was crushed, both shattering explosively.
Thanks Mom, Chara murmured softly. After seeing that in Alphys' anime stash, I'd have kicked myself all the way back to the grave if that reference hadn't been made.
Toriel chuckled softly. "Anything for my children," she murmured softly, letting her power start to subside. "Anything..."