Description
Icehollow or Icerahn, is the 14th dimension or realm. It became Herobrine's homeworld/exiled land after he was defeated in the Second Great War since he was revived there.
Icerahn is a cold realm, with an abundance of snow and ice. The sun never rises, and there are arches of crystal.
Legend
During the Second Great war Herobrine was killed, but not completely. His soul was transferred to a "Phylactery" or a "Soul Crystal" in Icerahn.
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.