Ervyll was the king of Verden, one of the minor realms in the Northern Kingdoms. His son Kistrin was once a prospective suitor for Ciri, but the young princess refused and ran away and Queen Calanthe formally called off the engagement for other reasons.
Ervyll was married to Eliza, the sister of Freixenet, who'd been the baron of Hamm before being cursed and turned into a bird. However, Ervyll showed enough concern for his wife, who'd been trying everything she heard of to try and lift the curse, that when Geralt passed through he paid the witcher's asking price to lift it. The king then employed Freixenet as one of his governors.
At one point, after Roegner of Ebbing had died, Ervyll proposed to Calanthe but she turned him down.
He was painted as possibly a pedophile as well as a racist. He had designs on Brokilon and both Ervyll and Eithné, the dryad queen, mutually hated each other. He went so far as offering rewards for dryad scalps and having some of his soldiers attack merchant caravans to try to frame it as the dryads' doing.
During the Second Nilfgaard War, Ervyll betrayed his allies in the North, swearing fealty to Nilfgaard. His son, Prince Kistrin, opposed this move, and started a guerrilla uprising, aiming to depose his father and remove any Nilfgaardian influence from the kingdom. Eventually, they were successful, and Kistrin drove his father out, seizing the throne. Ervyll was assassinated soon after.