Alright...so..uhm...uh...well...anyway...just...W.T.F

It's okay, Satori.
This one kid sacrificed his limbs and little brother to open this gate. A pair of panties is nothing.

Description from the pixiv page:

Satori: "I really didn't want to have to use these, but I have no choice."

To claim victory for Subterranean Animism in the final round of the tournament, Satori opens a sealed door hidden deep in the underground! Inside the door was a black ball, from which appeared a white piece of cloth.

Yuugi: "No way... Are those...panties?"
Orin: "You know about this Yuugi!?
Yuugi: "Yeah. It's a terrifying weapon made by the gods."
Okuu: "W-What did you say!?"
Yuugi: "They were made long ago, in the middle of a large dispute."
Kisume: "My orichalcum bucket took huge damage from that thing."
Yuugi: "Us oni were powerless against the humans that wielded it, and a magician of overwhelming power was even sealed away. The rest of the youkai were soon defeated by the lunarians' latest model of panties."
Parsee: "Even the bridge I was living in got contaminated..."
Satori: "Mission Start. The enemy are the Onbashirans!"

So the lunarians invented panties in order to defeat Onis?

So Reisen only being good for her sex appeal was the secret weapon in this conflict.

Thanks, Eclipse5632.
I'm always wondering about this Pixiv tag that someone added to his images: 'はいてたら人類滅亡' (If the artist draws 'wearing panties' images, mankind will perish.)
Now it makes sense.

Actual panties in Akashio's art? Madness! His girls never wear underwear!

Madcat6204 said:
Actual panties in Akashio's art? Madness! His girls never wear underwear!

And now we know why.

kunangkuadrat said:
It's okay, Satori.
This one kid sacrificed his limbs and little brother to open this gate. A pair of panties is nothing.

Hahahaha, you sir get an internet.