The Light Elves of Alfheim are ruled by Gwyn ap Nudd, and are amongst the most technologically-advanced Mythborn species. Their kingdom is the most powerful in the Summer Court, leading to jealousy from Titania's Fey.
The Dark Elves used to be part of the Winter Court of Fairie. However, due to Mab not trusting the Elves because of their military might rivaling her own, they were always kept from getting too close to places of power. Growing resentful, they ultimately joined the Titan Erlik and split from the rest of Fairie, settling in the realm of Svartalfheim in Patala. Since then, the realm of the Dark Elves is the main gateway for the Titans into the Winter Court.