"Wow, you... You really mean it? You'd give up your own soul for me, Frisk?" Asriel looks stunned by your sudden offer, eyes wide, but glimmering with hope and gratitude.
You reassure him that you would, that you'd do anything to make sure he never has to go back to being a flower- that stabilizing his form with a soul now before he reverts should be able to sustain it. He can't stop himself from smiling now, wiping away a few tears and nodding. "A-alright! I'm ready, then!"
.
.
.
After a good while, you realize you have no idea how to actually offer your soul up to him.
.
.
.
The silence is beginning to get awkward. Asriel's smile is faltering.
.
.
.
"...It's alright. I guess it was too much to hope for, huh?" He tries to play it off with a chuckle, but you can hear the sadness and disappointment in his voice. It's tugging at your heartstrings.
You are determined to get your soul into him. So, maybe if you just...
You tell Asriel to open his mouth.
"H-huh? Um, alright... Why am I- M-mmff?!" You cut off his sentence by cramming your hands into his mouth. You can feel his tongue, warm and wet, pushing against them, instinctively trying to push your hands back out. That won't do at all.
You reassure Asriel that it'll be fine. He just needs to trust you. This is the best way to do it, giving yourself to him completely so that he can sustain himself. His expression calms somewhat, and he makes a muffled noise in assent, and starts helping out- your hands slip to the back of his mouth, and slide down into his throat as he starts swallowing.
You'd suspected this might be possible- monster bodies aren't physical, after all, so there's no need for them to necessarily follow physics for how a body should work. Still, it's an interesting sight to see, the way he's stretching out to accept your arms. You can feel his throat, pulsing and swallowing around them as they're pulled deeper into his body.
Very soon, your head is at the verge of his lips as well. While he can, Asriel gives you a grateful look, silently thanking you, before the young goat boy swallows again, and starts pulling your head into his jaws as well. You must taste good, too, because his tongue seems to be all too eagerly tasting you, dragging across your features for seemingly entirely too long, before another gulp pulls you down into his throat... You're not entirely sure how to feel about it, but you're glad he seems happy at least.
At the very least, the process isn't as unpleasant as you were worried it might be. The throat squeezing around you, while tight, seems rather gentle. You'd almost liken it to an embrace. A squishy, wet embrace that's slowly enveloping more and more of your body, instead of just a pair of arms.
You can feel your hands pressing against something- they must have hit his stomach. And as more of you is being gulped down, you must be starting to stretch him out, giving him a slowly swelling stomach.
Or, maybe not so slowly after all. Asriel seems to have reached your waist now, and there seems to be little enough left now that he can turn his head up, lifting your legs up into the air, and letting gravity help him devour his best friend now.
You can feel every swallow, helping to ease you along down his throat, as more and more of your body fills his stomach. The air is hot and humid, and it seems to be dwindling rather rapidly- you feel a little light-headed. But you suppose that's fine. Thinking about it now, even if sitting in an empty stomach is okay, there are later parts of this that you'd rather not be conscious for.
A few more gulps, and the last of your body is sealed within the warmth and tightness of Asriel's. You're deposited down into his stomach, curling up in the tight confines, and you can feel something, Asriel's hands most likely, pressing against you through the squishy walls surrounding you.
"H-heh... There was probably an easier way to do that, Frisk..." You can hear his voice, as well as feel it, echoing slightly, seeming to be coming from all around you. "But... thank you."
You assure him that it's no big deal, squirming around slightly, to try and get a bit more comfortable. You can feel yourself being squeezed from the outside, again, Asriel seeming to be hugging his belly to himself.  "Mmmm, I can feel you wriggling..." It's a familiar line, but coming from the real Asriel, now, it sounds so much more cheerful and innocent. You keep moving around, long enough to get nice and cozy, before allowing yourself to pass out.
Upon awakening, you find yourself no longer within a cramped stomach. Nor within your own body, for that matter- looking through your eyes- or through Asriel's eyes, you quickly realize- you can see your own stomach, now a huge, furry orb, stretched out in front of you, resting in your lap. You poke at it a bit, and start to chuckle. No, Asriel starts to chuckle.
"It's kind of weird, isn't it? Sharing a body with someone else... Seeing yourself like that... Even if you can't quite see yourself right now, heh." That's right, Asriel would know what this is like. It isn't the first time, after all. "It's... nice, really. Your soul... your presence, feels so much... warmer and kinder than Chara's ever did. With you guiding me... maybe I'll be able to make up for all the awful things I've done."
You assure Asriel that it wasn't his fault- he doesn't need to make up for anything, and everything will be fine.  Not with words- just the feeling from your soul is enough to reassure him. You can sense his feelings as well- his determination to make the most of your bonding with him.
"Though, ah... Maybe we should wait a bit before going back. This would... Probably be, um. Awkward to explain. Not to mention heavy to carry..." Asriel prods at his belly a bit, as it churns softly, seeming to be rather squishy.
You agree to wait for a bit before returning to the others.