Is This the Final Domain? No... Is It an Ark?

In a boundless expanse of shimmering light, radiance gathered and two figures appeared, both wreathed in rosy light. One had pink hair and black wings; the other's sky-blue hair fluttered in the air.

"Is this the Final Domain?"

Remiel Dan surveyed the unfamiliar world before her. One half lay under an eerie daylight, while a sinister purple curtain shrouded the other in night.

Remiel Dan folded her wings and descended, her feet landing on a sea of sand. What a strange place!

The world felt boundless and alien, like a formless chaos. Yet the sand beneath her feet felt real, just like a beach by the sea.

Was this the Final Domain she had sought as its gatekeeper?

And what was this wall in front of her?

"Sister Dan, why are you spacing out?" Griseo leaned over curiously.

"Your name is Griseo... right?" Remiel Dan suddenly asked, recalling Kevin's words.

"Mm. I'm Griseo." Griseo nodded.

"Griseo, is this the Final Domain? Can you tell me where Ether ultimately comes from?" Remiel Dan couldn't hide the curiosity in her eyes.

"What Final Domain? What do you mean, ultimately?"

Griseo looked at Remiel Dan in confusion. Had this big sister misunderstood something again?

Oh well. I'll show her.

Used to being misunderstood, Griseo got to work under Remiel Dan's gaze. She searched the wall in front of them, checking something... Finally, she pressed one of its stones and said:

"Marvin, I'm back. Drop the disguise!"

[Command accepted. Security verification activated... What's the hardest thing in the world?]

The unfamiliar mechanical voice startled Remiel Dan. What was that?

But what happened next was truly astonishing.

"Grandma Mobius's mouth," Griseo answered softly.

[Correct answer.]

A mechanical chime sounded. The wall changed color, and a metallic sheen appeared before Remiel Dan's