Icerahn is a cold realm with a lot of snow and ice. There is no sun and there are arches of crystal everywhere.
Icehollow is home to many dangerous creatures such as the Frostwisp, whose venom puts you to sleep, and can be cured only with a Spellcrush or Cleansing potion. Therefore, there are no forms of life other than hostile monsters.
The Knights of Aetheria soon found it along with a bunch of mobs performing a ritual which, once completed, would revive Herobrine completely. The Knights killed the mobs and then tried to destroy the crystal, but it was indestructible to them. So they decided to make a 24/7 tower post there to kill all mobs that would try to come there to revive their master.
For a long time, the mobs were slowly forced back further and further in all the dimensions. The mobs then were scattered to the four corners of the Overworld. However, they regrouped and rejoined each other, slowly expanding in number and strength until they were strong enough to launch an all-out attack on the post.
On that fateful day, many Knights were lost under combat. Once the mobs controlled the area in Icerahn, they revived Herobrine fully. With himself back in power, Herobrine began to reattempt his conquest of all of Aetheria.