The King of Hybern was the ruler of the island of Hybern. He allied himself with the Mortal Queens and the Spring Court in order to wage war against the rest of Prythian in a bid to reclaim the human territories for the High Fae. While he was able to break Feyre and Rhysand's bargain, he was not able to break their mating bond.
While considered handsome by human standards, the King had ruddy skin and inky black hair framing a rather plain face compared to most High Fae. He wore sturdy, practical clothing and had a muscular build.
The king was cunning and brutal, more than willing to sacrifice the lives of soldiers and even relatives in order to accomplish his goals. He was also willing to negotiate peacefully, as shown with the mortal queens and Tamlin. It is likely that the king always intended to betray them, however. He was also incredibly intelligent, tricking the Weaver of the Wood into letting her guard down so that he could kill her, then using the Archeron's father to ensure that Nesta did not kill him immediately.
The king was an avid student of the magical arts, and kept many powerful spells within a spellbook that he carried at all times. It was he who learned where to find the cauldron and how to reassemble it and use it as a weapon, and he was always eager to get his hands on a new piece of magic.
It is unknown when or how the king ascended to the throne of Hybern, but he was the ruler at the time of the human rebellion and fought on the side of the loyalists. When the loyalists were defeated, Hybern was forced by the treaty to free their human slaves, although Amarantha disobeyed the order, but did not give up any territory to the victors. For the next five hundred years the king nurtured his people's hatred and sense of injustice, preparing for an opportunity to wage a second war in which he would emerge the victor. He sent Amarantha to Prythian as an ambassador more the fifty years before the events of A Court of Thorns and Roses, ordering her to find allies on the island. Instead, Amarantha tricked and enslaved the High Lords of Prythian and set herself up as a queen. The king was displeased by this development, but allowed Amarantha to rule as an experiment to see how well he could subjugate other faerie rulers when he went to war. After Amarantha was defeated, he gave orders for his troops to raid various temples around Prythian and steal the feet of the cauldron, killing the priestesses so there would be no witnesses. He resurrected the human general Jurian to serve as an ambassador to the mortal queens, offering them immortality for servitude, and made a deal with Tamlin to break Feyre's bond with Rhysand in exchange for information and aid during the war.
He caught Feyre and the other Night Court members in a trap as they attempted to destroy the cauldron. There he revealed that he had captured Feyre's sisters, and the proceeded to place them in the cauldron to test its effects before allowing the queens to enter it and become fae. He also fulfilled his promise to Tamlin by severing the pact Feyre and Rhysand had, but was unable to sever their mating bond as he was unaware of its existence.
Later, the king sent his niece and nephew Dagdan and Brannagh to the Spring Court in order to determine where the wall was weakest. When Dagdan and Brannagh were killed by Feyre, he invaded the Spring Court as vengeance for their deaths and proceeded to destroy the wall using the cauldron. He waged war against the other courts of Prythian, with most battles being fought in the Summer Court or the human territory. During the final battle of the war, he unveiled the cauldron's magical might, annihilating the Illyrian flying legions and killing the Bone Carver in the proccess. When the Weaver fought her way into the Hybern battle camp as a distraction for Feyre and Amren, the king confronted and killed her by tricking her into lowering her guard, then snapping her neck and throwing her to his dogs. He then sensed Nesta's presence and went to confront her. Before he did so, he winnowed to Nesta's father and captured him, so that Nesta could not kill him immediately. He then snapped her father's neck, breaking her spirit momentarily. A lengthy battle ensued, with the king defeating both Cassian and Nesta. Before he could land the final blow, however, Elain stabbed him through the back of the neck, and soon after Nesta finished beheading him, killing him along with Elain.. After his death, the remaining Hybern forces continued to fight on, driven by the bloodlust and sense of outrage instilled in them since birth, until Amren entered the cauldron and wiped them out to the last faerie. The state of Hybern after the death of their king is unknown at this time.