Underneath Mount Ebott is a surprisingly large cavern complex. On the western edge, there's a ruined town built from purple brick. On the western edge of that, sunlight from cracks far above illuminates a patch of golden flowers, surrounded by pillars.
The presumable source of the fireball approaches.
"What a terrible creature, torturing such a poor, innocent youth… Ah, do not be afraid, my child."
"I am Toriel, caretaker of the Ruins. I pass through this place every day to see if anyone has fallen down."
"You are the first human to come here in a long time. I will do my best to ensure your protection during your time here. Come! I will guide you through the catacombs."
"I could heal you?"
Toriel looks through her pockets.
"I could give you my spare cell phone, but I think those are cheaper on the surface. I could stay at a distance and wait for you to take some time to think. I could lead you at a distance and you could watch my exact steps, instead of taking my word on how to avoid hazards."
"All right. Telephones are devices that are used to send sound to other telephones that are far away, hence the name. Early telephones connected to each other with wires, but cell phones are portable and don't need that kind of connection. To make a phone call, you enter the number for the person you want to call. To reach me, you would enter 23549, then press the call button. After that, my phone would ring, and I would answer the call."
"Yes."
Toriel gets out a pair of black devices and sets one on the ground, then steps away from it a bit. She unfolds the other one, and shows Blai the buttons. She then presses buttons on it. Each button makes a different beeping noise. Shortly after she presses the sixth button, the device on the ground starts ringing.
"There are two kinds of people: humans and monsters. Both kinds have souls. Humans have strong souls, like yours, which come in a range of colors. Monsters have much weaker souls which are white and look like upside-down versions of yours. Humans are animals that are people, so they have sturdy physical bodies made mostly of water, but their magic doesn't flow easily, and they have to go years between uses. Monsters are spells that are people, so we have magic bodies and can do magic so often we sometimes do it just for fun."
"The monsters in the Ruins are mostly not that hostile. I'd worry more that if I didn't keep an eye on the ones who saw you, someone would eventually sneak past me to the exit and try to turn you in for a bounty, or such. Also, if you set up in an abandoned building and avoided other people … well, you'd be in an abandoned building avoiding other people. Even if you can conjure your own food, it would likely be difficult and unpleasant to address all of your needs on your own."
Toriel leads Blai through the gateway and into the first room. It has an arrangement of 6 pressure plates leading to a lever. She steps on the outer 4 and pulls the lever, opening the door to the next room.
"Allow me to educate you in the operation of the Ruins. The Ruins are full of puzzles. Ancient fusions between diversions and doorkeys. One must solve them to move from room to room. Please adjust yourself to the sight of them."
He could plausibly cross before an Air Bubble expired. The water might even be shallow enough that his head would be above water if he stood on the floor, though it's hard to say. The water is running perpendicular to his direction of travel, though, so walking might be difficult.
Also, the gaps between the bridge and the wall are in many parts narrow enough that he could keep a hand on both a non-spiky portion of the bridge and on the wall simultaneously.
They exit on the other side of the false floor. There's a rock by a pressure plate and spikes blocking the way. Toriel slides the rock onto the plate to make the spikes retract.
"Up ahead is another puzzle where you need to walk a specific path, or else you fall in a pit and the only route out sends you to the start. I have it memorized."
"I told them the king would try to kill them. They were children. It's a lot to explain to a child. They weren't the seventh."
"Also, I wouldn't summarize the situation that way. There is no ritual that uses seven human souls to annihilate humanity. Absorbing human souls simply makes a monster powerful, and seven is estimated to grant godlike power and be sufficient to take down the barrier. You may call me an optimist, but I do not think the king has the will to follow through on his statements."
They head through what seems awfully like multiple versions of the same room with lighting from different directions. Toriel flips some switches.
Finally, they make it to a brick building whose door has a regular lock and a handle. There's a leafless tree and another star outside the house.
"We're home!"
"And any other plates you have conveniently available. I usually make pies and filled rolls and cheese and fruit if I don't have a bowl or pot for a soup - I can make the crust hold the pies together - but I can add soup if you have a vessel. Plus your snails and butterscotch cinnamon pie."
And then there is a feast! He underpowers the spell a little - it can feed fifteen, and even if she gives away a lot of snails they won't get through that much even stuffing themselves. But there's her pie in her pie dish, and lots of human food, little pies sweet and savory, rolls and cheeses in heaps, a platter of grapes and apples and pears and blueberries and cherries and strawberries and oranges and pomegranates and peaches. The pot fills with beans and spinach and dumplings and onions and broth.
"One takes turns - the player with the white pieces always plays first - and moves exactly one, each turn, according to how each kind moves -" He lists all the movement patterns. "- and captures diagonally like so," he concludes, regarding pawns. "The object is to place the king in a situation where it cannot avoid capture; because that is the object of the game you may never move to put your king in a threatened position, that being just an unnecessarily elaborate resignation."