This was probably Frisk's last time through the underground. It wasn't because he wanted to stop, far from it. It's was that he had to. When he first met Asriel, he was filled with what seemed like infinite determination to save him. But nothing is truly infinite, and Frisk slowly came to realize that. He learned that resetting took lots of Determination, and that every body has a limited amount that it can hold. Even his. He at first thought this wasn't so bad, until he found some of Alphys' notes on the subject. He learned that every muscle in the body, every organ, every beat of the heart, relies on Determination. Without it, the body would cease to function.
Frisk noticed it happening slowly. Aches in his body that butterscotch-cinnamon pie and a rest at the inn wouldn't fix. Intense cramps and pain. Nosebleeds. Vomiting. Coughing up blood. And with every reset, it got worse. Sometimes it was so bad, he had to take a break for a few hours before continuing on. Before, he had hid it from his now-precious friends, but he realized that this time, he couldn't afford to.
"Howdy! You're new to the underground, aren'tcha?"
Whenever he saw Flowey pop up in that barren room, Frisk felt a bittersweet mix of joy and sadness. The thought of saving him was one of the few things keeping him going. But there was no time for that now.
"Do you want me to share some LOVE with you?" Flowey asked as he performed an innocuous wink.
In the beginning, these slow, almost harmless pellets had been easy for Frisk to dodge. But Frisk wasn't exactly in peak condition. Touching any one of those bullets would be enough to finish him off, and he wasn't sure he had enough determination left to LOAD. He was on his last life.
The first wave went well enough, although it was a little close for comfort. The second he just barely dodged, and on the third, one of the pellets grazed his hair.
"You know what's going on here, don't you?"
As the room filled with a sphere of bullets pointed towards him, Frisk ironically felt relieved. He had done this so many times that he could count the seconds.
Three... Two... One...
As if on cue, a fireball appeared out of the darkness and knocked Flowey out of the way.
"What a horrible creature, torturing such a poor innocent youth! Greetings, I am Toriel, caretaker of the Ruins. Oh dear, it seems you are hurt. Let me heal you up a bit..." She pointed her hands at Frisk and closed her eyes, concentrating. Nothing. After a few moments, she opened her eyes. "This is strange. That should have worked..." Frisk considered telling her about Flowey, about the resets... About his situation. But he decided to hold off on that for now. He didn't want to make her worry too much yet.
For now, he simply said, "I don't feel good, can you carry me?"
"Why, of course, child. Here, climb onto my shoulders." She offered Frisk a hand, and with that, they were off. The walk through the ruins was quiet and tranquil. There were no puzzles this time, not that they gave Frisk much trouble anymore. About halfway to Toriel's home, he fell asleep.
When Frisk woke up, he could instantly tell where he was. The familiar smell of pie left no doubt in his mind. He tried to sit up, but his whole body howled in pain, and he fell to the floor. It had never been this bad before. It was like nails were being driven into every inch of his body. He cried out.
Toriel burst into the room within moments. "Child?!"
Between his screams of pure agony she could barely make out, "M-mom, it hurts! It hurts!" She could see tears streaming down his face. After trying and failing to heal him with magic, Toriel didn't know what to do. Crying, she held Frisk in her arms, and did her best to comfort him. After a few moments, his groans faded into an exhausted pant. While Frisk's crying eventually stopped, Toriel's only intensified.
"I thought I was going to lose you." She sobbed.
"Sorry for scaring you.." Frisk croaked hoarsely. "It... It looks like I can't stay here much longer."
"What are you talking about?!" she yelled in disbelief. "Do you honestly think I'm going to let you leave with the condition you're in? You're staying right here until you get better!"
"I don't think you can help me with this, sorry. I don't think anyone can." Frisk wiped away some drool from the corner of his mouth. "I can't tell you why, but I need to get to the surface. I promise, it'll make sense by the end of this. Trust me."
"...Well. If you insist on going, I guess I can't stop you. But I am most certainly not going to let you go by yourself. I'm coming with you." Frisk was surprised and overjoyed. This was the first time that Toriel had ever agreed to leave the Ruins with him.
He hugged her and started to cry. Not out of pain, but out of happiness.