Icepeak was a kingdom made up entirely of ice, located somehwere in the north, which was ruled by Nekron and his mother, Julianna.
Before the war
At the beginning of Fire & Ice, the female narrator states that Nekron used his powers to take control of Icepeak, implying that he did not form the kingdom and was not the only ruler.
War
Once Nekron came to power in Icepeak, he became more power-hungry and set out to conquer all of the world. Using his supernatural powers, Nekron was able to physically move and expand Icepeak. He pushed through Larn's village and made his way towards Firekeep, the last free corner of the land. However, when Darkwolf finally killed Nekron atop of the skull tower, Nekron's death throes casued Icepeak to cross the river adacent to Firekeep, forcing King Jarrol to release a flow of lave, which flooded across the jungle and then finally destroyed Icepeak, which collapsed when the lava destroyed its ground structures.
Post-war
With Icepeak destroyed in the Battle of Icepeak, Firekeep was now free to take control of what remained.