The War of the Elves and the Giants

Lengthy war fought between The Old Empire and The Giants.
In the years leading up to the war, more and more elves had settled in the lands north of the Caltsee , lands which the giants considered to be theirs. A Storm King emerged and called the giants to arms against the Empire. This was Gornningr, the first such Storm King. Gornningr roused the giants to attack the Imperial colonies north of the Caltsee, and in doing so cut off Mount Celestis and the First Library.
The elven settlers were slaughtered in the giants' first, entirely unexpected attacks. Then the giants gathered for a massive offensive around the eastern coast of the Caltsee, through what is now Sigsland and into the heart of the Empire while a second army of giants, sailing their enormous stone Giantships and supported by the Cloud Giants' Cloud Castles crossed the Caltsee and invaded Grefensee, Prunzinn and what is now the Grand Duchy of Oldbruggen.
The two northern giant armies met in Erbann, regrouped and divided into three armies. One moved southeast to invade Mitland, one invaded Zenden to the south and the third crossed the Great River and moved towards Frincia. The Elves could do nothing against this onslaught. Every major battle was a crushing defeat for the Imperial forces.
The southern Giants opened up a second front in the war by crossing first the Desert of Bones and then the Celestial Sea to invade Savinn, Lubriunn, Bohennin and Synoppeia.
The Elves fled from the invading giants, into the forests and particularly into the caves and underground halls of the Dwarves. From there, they began to regroup and started the fightback. This phase of the war - the 'Hidden War' - lasted for three centuries. The giants controlled most of the lands previously claimed by the Empire, but they were never safe from Elven fighters striking from secret bases.
During this period, humans began to migrate into Ettrea from the west, crossing the Atlan Mountains. The Emperor Ishbahel, who by this time had succeeded his father Nathah, offered citizenship to any humans who would swear allegiance to the Empire and pledge to fight the giants. Many elves were sceptical and even suspicious of humans, but their courage, military skill and sheer numbers were to prove decisive in turning the tide of the war against the Giants.
The first major battle featuring large numbers of human soldiers (as well as elves, dwarves and gnomes) was the Battle of Elbinn. The giant army was routed, and the southern giants played no further part in the war. This battle marked the turning point in the entire war. After the Battle of Elbinn, the Empire was always on the front foot, while the giants retreated into their strongholds and their cloud castles. And even those cloud castles proved to be less secure than they had been after Bilhalel's creation of the Crystal Ships.
A cadre of elven archmages slew Gornningr when the Storm King was sailing from Giantskeep to Miltrovin. Although Gornningr's lieutenant Heigmundr claimed the title of Storm King, and was able to hold the giants together, he was not able to reverse the tide of the war. After a series of defeats, Helbrodda, Queen of the Fire Giants withdrew her people from the war. After this, the giants would never again win a major battle.
Giant castles and holdfasts fell quickly as fewer and fewer giants faced more and more elf, dwarf, gnome and especially human soldiers. Elven mages and archmages had spent the best part of three hundred years perfecting new destructive magic, and the giants had no defence against it. Heigmundr withdrew to the mighty citadel of Giantskeep, most of the last remaining cloud giant castles left Ettrea altogether and most of the rest of the giants fled back to the north or into Viltland. Heigmundr was killed by another giant. There was no other Storm King, and it was left to another storm giant warlord, Byleistr to negotiate the giants' surrender.
Under the terms of the surrender, the giants agreed to leave Ettrea completely and retreat to their lands in the far north and the far south.
The centuries of war changed the Empire fundamentally. When the war began, there were no humans in Ettrea. By the end of the war, humans made up almost two thirds of the Empire's citizens. The war also raised the status of dwarves and gnomes in the Empire.