Astral Challenge is a in Heaven's Call, a mod for The Binding of Isaac.

Astral Challenge rooms can spawn in Chapter 2 and 3 with a 10% chance, and in Chapter 5 with a 30% chance. They can appear once per type, per run.

A astral item or a Planetarium item will always spawn in the room. Collecting or absorbing the item will trigger a battle against an astral boss. In the early chapters, this summons an outer planet boss, while in Chapter 5 it summons an inner planet boss.
Attempting to flee the fight will cause the boss to teleport along with Isaac.

If Double or Nothing is unlocked, stars will appear in the room the first time it is entered. Collecting all of them will make the boss battle more difficult but will also grant Isaac a Holy Card use. When the boss is defeated, an extra item from a random pool will be rewarded.
Getting hit during the battle will prevent the extra reward from appearing and remove the item originally spawned in the room from Isaac's inventory.

Double or Nothing mode unlock: Complete Tower of Babel challenge on ascended difficulty.

The following special synergies and interactions for Astral Challenge have been documented.

* Note that not all possible synergies and interactions are listed here.

• Download Heaven's Call from the Steam Workshop.