The Null Moon was one of two satelites that orbited Dominaria. It is also called Glimmer Moon and was originally known as the Null Sphere. It was originally a Thran base designed to control the empire's artifact warriors, but during the Thran Civil War Yawgmoth took control of it, using it to turn the artifact creatures on their masters. Later, he tried to use it to protect Halcyon and his Phyrexian forces from his stonecharger weapons. However, the sphere's operators rebelled against Yawgmoth, sacrificing themselves to jettison the sphere into orbit. This lowered Yawgmoth's protective magic, meaning that Yawgmoth's army was also wiped out by his weapons.
After the war, the Null Sphere remained in orbit for millennia, its orginal purpose eventually being forgotten. The only major event it was involved in was when it served as the venue for a planeswalkers' summit called by Faralyn, which descended into violence and began the Planeswalkers' War. Otherwise, the new moon was little more than a curiosity.
However, 9000 years after it was thrown into orbit, the Null Moon again played a role in defeating Yawgmoth. Since its launch into orbit, the moon had begun building up large amounts of white mana, which Gerrard Capashen believed could be used to slay the now-godike Yawgmoth. Gerrard took Weatherlight up to the far side of the moon and had the ship punch straight through it, cracking it like an egg and channeling the released mana through Weatherlight into Yawgmoth. This caused massive damage to Yawgmoth, but ultimately failed to kill him and only resulted in the death of Weatherlight's consciousness. The Null Moon itself survived, but was left with a large hole and no longer glimmered as the white mana that caused that effect was no longer inside it.