In the far reaches beyond the known cosmos, a group of creatures known as the Gemihive originate. They were essentially living breeding grounds
for the Crystal Sparklers, and were created by the Presiding Gods of the Moon in order to do their bidding in places the Gods couldn't reach. After the Gemihives
were assembled, the Moon Lord would then assign them a planet and task them to complete their mission. Initially, the Presiding Gods decided to give the Gemihives
the ability to transport themselves between dimensions at will, however, this led to too many cases of Gemihives straying from their duties, and leaving the planet
they were assigned to. In order to remedy this, the Moon Lord and the other Presiding Gods placed an eternal spell on the Gemihives to prevent them from transporting
between dimensions until their missions were complete, as well as enchanting them with a sort of "third eye" which allowed the Gods to see through their bodies.
The Moon Lord then used the enchanted "Third Eye" in order to peer beyond his own omnipotent sight and locate the stray Gemihives. It turns out that the
strays had actually found their very own planet, and are blessing it's ground to become fertile once more so as to start a sort of Gemihive colony. The Moon Lord was not
going to let any of that happen, however. Using his magics, he opened the "Third Eye" of one of the strays especially wide, before firing a laser into that "Eye". This caused
the laser to go through said eye and appear at the Gemihive's actual position. The Laser then obliterated the colony, and caused the entire planet to combust into infinite
numbers of crystal shards. As these shards are exposed to the vacuum of space, they began to turn light blue, while others that are much denser turn white. Eventually,
when our Terrarian kills the Wall of Flesh and frees the Souls of Light in his world, a portion of the crystal shards in space are attracted by Terraria's gravitational
strength, and since the Souls of Light are present and free, were able to collide into the earth and spread out throughout the land, forming the Hallow. However, not all
stray Gemihives were dead. Some larger crystal shards in the hallow began to reform into the hives they once were, with the intention of restarting their destroyed
colony in your world, very slowly but surely.