A previously isolated world.
It was a prison, but for Merem it was also a place of comfort like the womb. After acquiring his freedom, he peered at the outside world through this place.
In the past, many of his “wishes” in his imaginations were born from the theme of the Temple.1
The only thing he loved and longed for from the outside world was the Crimson Moon, as a result, since then he purposefully avoided imagining things with Moon themes.

Translator’s Notes
  1. ^ This is more obvious if you look at Merem’s concept art, the ubiquitous “Temple mosaic pattern” is found on his clothes, trinkets, as well as the Four Great Demon Beasts. See Character material for more details.
Character Material: Prelude Explanation