The Kingdom of the Isles was a semi-autonomous kingdom located in the Western Isles of the Zarkan Kingdom, a set of islands off Zarkania's west coast, that was created in 1,000 BBY after the Duchy of the Isles received an increased amount of political autonomy from the Kingdom of Westland, the former rulers of the Duchy who had conquered and settled the Lordship of the Isles in 1,211 BBY, in exchange for recognition of Westland as the Suzerain of the Isles.
After Westland's crushing defeat in the Zarkanian War of Independence against their Zarkan Colony Duke Eric II decided the time was right to press his claim for becoming a fully-fledged kingdom, he sweetened the deal for Westland by promising to swear continued fealty to Westland. Westland, having just lost their southern colonial vassal states, agreed to the proposal, and Duke Eric II became King Eric I (Though in Historiography, was often referred to as Duke Eric II even after he became king, for simplicity)
The Isles' fealty to Westland only lasted another 11 years, until 989 BBY, when Eric II, under pressure from King Thomas Zarkan I, switched his fealty from Westland to the Zarkan Kingdom. Aside from Zarkan's pressure, Eric was convinced that making a new oath to the Zarkanians would be best for his own realm, since Zarkania was now the new strongest power in the region, and Westland could no longer guarantee his kingdom's security.
This was followed by almost 140 years of relative peace in the isles. This peace, however, ended abruptly for a time in 862 BBY. King Eric IV, the great-great-grandson of Eric II, a sympathiser of Westland, and desiring a return of the Isles' sovereignty, retracted his oath of fealty to the Zarkan Kingdom that he had taken the throne in 866 BBY. Eric IV was a popular figure amongst Isles nationalists for this act. Nationalists of the Isles had, by this point, spent decades lamenting the isles' status as a defeated vassal of the Zarkan Kingdom, and desiring a return to the semi-autonomy of Westland's rule, they also considered Eric II, Jorunn I and Eric III weak for their willingness to bend the knee to Zarkanian overlordship with little resistance.
Angered at this betrayal, and believing Eric IV was planning to collude with Westland to invade the Zarkan Kingdom, the Zarkanians declared war on the Isles, beginning the War of the Isles. The Isles' fleet was overwhelmed by the Royal Zarkan Navy, and the Zarkan Kingdom commenced naval landings on multiple isles. In three weeks all of the isles were subjugated or besieged, and the capital island, Greenisle, had been conquered.
In response to this betrayal, the Zarkan Kingdom deposed Eric IV, who was publicly humiliated by being forced to kneel before the Zarkanian king and swear his fealty. Eric IV was then exiled to a deserted island, of which he was mockingly proclaimed "king", where he died of disease.
With Eric IV removed, the Zarkanian officials decided to extend a show of good faith towards the Isles and had Eric IV's son, Eric V, crowned as Grand Duke of the new Grand Duchy of the Isles. The Grand Duchy was, however, very much a puppet of the Zarkan Kingdom, even more so than the Kingdom of the Isles had been.