The Shadow King was the monarch of the Shadow Kin and ruler of the Underneath. (TV: For Tonight We Might Die) The Shadow King ruled from the Shadow Palace. (TV: Co-Owner of a Lonely Heart)
Corakinus was the Shadow King until he was defeated by April MacLean. She then succeeded him as Shadow King, but lost the power after Corakinus broke the link between him and April. (TV: Brave-ish Heart) Charlie Smith briefly took on the role after killing April and Corakinus, and shortly thereafter, committed genocide on the Shadow Kin. (TV: The Lost)