Though Iarlath was common folk, he became a court member and even the right hand man of the Unseelie King—honors usually only given to gentry faeries. He and the warlock Malcolm Fade became acquainted some time in the 19th century when the King became indebted to him.
After the attack on the Los Angeles Institute, he allied himself with Malcolm. He helped the warlock in exchange for the death of his many enemies in the Seelie Court, who would be murdered by his followers.
Because of the mysterious killing of faeries—and mundanes, though they did not care for this—in 2012, the fey started an investigation. For appearances' sake, Iarlath involved himself in the investigation. As part of his deal with Malcolm, he was confident that the killings would not be traced back to him.
The Unseelie Court worked to uncover clues about their deaths, studying and translating the demonic markings inscribed on the victims' bodies before they crumbled to ashes. They discovered that the markings were a language from an ancient time of Faerie, back when the fey were more closely tied to their demonic ancestry. Their scholars managed only to translate the first line before the Unseelie King forbade them from continuing, afraid of the power they could awaken.
Shunned by the rest of the Downworld because of the Cold Peace, the fey had no choice but to turn to the Nephilim—this was, however, part of the plan Malcolm concocted with Iarlath. They began sending Arthur Blackthorn letters regarding the murders, setting up a meeting and making them an offer that he would return Mark Blackthorn, who had been conscripted to the Wild Hunt for around five mortal years, to his family in exchange for their aid. Arthur, almost out of his mind by then, ignored the letters, not taking it too seriously.
Eventually, Iarlath himself went to the Institute, along with Mark and Kieran of the Hunt, to try and finalize the deal. He told them that they would give them the translated first line and, through it, they must find the murderer and turn him/her over to their King to face trial. As they expected, the Shadowhunters agreed to investigate for them, in secret from the rest of the Downworld and the Clave, in exchange for Mark, who would help with the investigation for the Hunt—though by the end of it all, Mark would choose whom he would return to—and for Emma Carstairs, whose parents were also killed in the same manner. With both sides agreeing to the terms, Iarlath and Kieran left Mark at the Institute.
When Iarlath adamantly refused Mark's return to the Faerie, Kieran became suspicious of him. Kieran tailed him and eventually discovered his involvement with the killings and Malcolm's scheme against the Nephilim, threatening him at knifepoint to learn more. Kieran killed Iarlath after he gave him the information he needed.
Iarlath had dark green hair and yellow eyes. He somewhat resembled a tree, with branches growing like twisted horns from his head and his bark-like skin, his mouth looking like a slit on his face, his rough three fingered hands, and with thorns as nails.
Skills and abilities
- Faerie physiology: As a faerie, Iarlath had supernatural speed and swiftness—enough that he was able to outrun trained Shadowhunters.
- Faerie magic: Iarlath showed himself to be proficient in the magic of the Fair Folk; being capable of convincing mundane humans to give him their jewelry and he froze Emma's body to prevent her from running to Julian's aid. He was also able to capture his memories with magic and show them to others.