Sans:

Sans is, shall we say, distracted from how suddenly things have gone all sideways. It's more like the book is just there in his hand and everything outside of that isn't important. Then, a memory: W.D. Gaster, and what happened to him. Sans was there when Gaster fell into his creation. Gaster had gotten so wrapped up in science, in trying to corral magic beyond measure with scientific prowess, and Sans remembers now how the whole thing blew up in their faces, how Gaster fell into the Core and shattered across spacetime and was obliterated almost entirely from the memories of everyone.

It makes for a pretty good, sudden morality tale on the dangers of trying to use wild, powerful magic no matter how good your intentions.

Also, if you guys have the time, can I have a general summary of the kind of things Sans would have heard under the relic's thrall? You don't need to get super in-detail with it or anything, of course.

Papyrus:

The waves of despair and fear of losing everything were a surprise, and the way they jostled memories loose in a flood even moreso. He still doesn't remember joining the Reclaimers, but he's remembering so much else that he doesn't even make note of it.

Sans, of course, takes the chance to make a couple awful skeleton puns at him... seemingly mostly to point out that, just a couple hours ago, Papyrus would've been laughing along. Papyrus, of course, falls into familiar grumbling, the contrast any teller of bad jokes needs.

It's less surprising with each wave of awful feelings, and it's starting to seem that's all the defenses are - feelings. He holds steady, braced for it, and reassures the others (with Sayori's help) as they push on through illusions and hands and finally the heist.

It's suspenseful, watching Mira's mage hand at work, seeing Sans unusually still and focused on the job at hand....

Too focused, it turns out, emphasis hand.

Time seems to slow once Sans catches the book, which probably isn't a good sign but also isn't the kind of clear catastrophe people have described.

Papyrus moves to help Mira pull the book away from Sans, by pulling his brother away from the book... but honestly, he's less concerned about the relic's power than the others, and puts more efforts into talking at him. "Good job catching that book," he tells Sans while tugging him back, "but that's going in the phone, remember?? So Angus can use it to fix things??? Don't skip steps without warning us, Sans!"

Mira:

She watches all of these images, images that are painfully familiar. The worst thing is that a part of her—and it’s not small—wants these. But the longing and desire shifts and changes until they’re transformed into something else.

Anger. The Relic is using her brother, her friend, her pain and longing against her... and that makes her pretty damn angry. In a moment, that fleeting longing for a so-called perfect world is erased. She wants to see her brother happy, yes. She doesn’t want to be a pawn anymore. She doesn’t want to hurt anymore, but there’s something else that’s clearer in her mind.

“I don’t need a perfect world,” she tells the Relic firmly and with her head held high. “If I lived in a perfect world, I wouldn’t be the person I am. And I’m pretty proud of the person I am. Besides... who am I to make that decision for everyone? I won’t be that selfish. There’s no such thing as a perfect world, and I think we both know that.”

Besides... if she lived in a perfect world, she wouldn’t have met the other Reclaimers. The brothers she’s standing here with. Christine, kind and who’d offered her comfort. Sayori, whose words had bolstered them, whose smile is as bright as the sun. Spike and Church and Rose and the Literature Club, countless Reclaimers she’s seen and talked to, and especially— Monika. Monika, who’s been a steady presence, who’s made a place in Mira’s heart and won’t ever leave.

She wants to see Monika again, and that’s what ultimately solidifies the choice. Mira addresses the Relic once more.

“You might be offering me a perfect world, but this is the one I want to live in. It’s not perfect, but it’s more than good enough for me. I don’t need your power to make me happy, because I already am.”