Ferrum was the original Iron King and the first Iron fey. He was thrown off the royal chair by Machina and lived among the packrats, who he ruled. Ferrum was killed in the battle of The Iron Queen by Meghan Chase who he almost killed. Ferrum was the "false" Iron King who sought to destroy the Winter and Summer fey.
Ferrum was described as paranoid, crazy, angry, and sad. At first, Meghan thinks he's just a sad, angry old man, until she sees the evil inside.
In the first book he is described as being very, very old with metallic grey skin and white hair that reaches past his feet, and green eyes.