Stardust Fragment Block

It costs the same currency as your endgame gear, so building with it before the Moon Lord is down means going back to the Stardust Pillar for more. Decorate after the fighting is finished.

The blue and yellow crystal look plus the built in glow makes this the natural pick for anything meant to read as night sky or deep space. An observatory ceiling, a floor under a starry backdrop, or the trim on a summoner base all work. Since there is no wall counterpart, the block has to carry the room on its own.