Astral Challenge Room itself

Almost all Astral Challenge Rooms look the same (so the boss is a surprise!), they will all spawn with an item pedestal and an Astral Statue in the middle of the room.

The item on the pedestal belongs to the custom Astral Challenge Room Pool, it as a 65% chance of taking an item from the PLANETARUM POOL and a remaining 35% of taking one of the 13 zodiac items (12 zodiacs + Zodiac), the pool size is kinda small, so it's not that rare to reroll an item into itself. Any non hardcoded item spawned here will belong to this pool.

The Astral Statue is the one responsable for spawning the boss, to avoid the statue being triggered you can kill it with a CHAOS CARD.

The Astral Statue is triggered by either taking an item pedestal in the room or by bombing the statue, if the item is taken, a boss will spawn and nothing else, but if its bombed, it will also convert the item pedestals in the room into random pickups (6 to 10), the pickups spawned have high chances to be special ones, like golden versions, key pieces or even giga bombs (rare).

If you are skilled enough, you can try to blow up the statue and take the item in a frame perfect move.