Eochaid was apparently born to Mannan and lived almost since the beginning of time. Along with his siblings, he served as a Rider of Mannan, doing the bidding of the Unseelie King. In the old days, he hunted giants and monsters with his siblings.
In September 2012, Eochaid and his siblings were sent after the Blackthorn family to retrieve the Black Volume of the Dead for the Unseelie King. Eochaid, Karn, and Etarlam went after Kit Herondale and the twins Ty and Livvy Blackthorn in London.
During the ensuing battle with the children, Eochaid came face to face with Kit, whom he recognized, but he attacked him before he could ask him to elaborate. Before he could kill him, the Hunter Gwyn ap Nudd arrived to join the fight and protect the children. Gwyn threw Eochaid into the river. When Eochaid made it back to the surface, he was surprised to suddenly feel his brother Fal's death, which he relayed to his present siblings. When Gwyn suggested sounding the horn for Fal, Eochaid was insulted and tried to attack him but was stopped short by the Shadowhunter Diana Wrayburn. He left with his brothers to rejoin the others after this.
Eochaid and the remaining Riders went to the London Institute shortly after. He called Kieran a traitor upon seeing him with the other Shadowhunters and fought him and Mark Blackthorn, until Annabel Blackthorn appeared and threatened to reveal the Unseelie King's name. Eochaid then retreated with his family.
Like the rest of the Riders, Eochaid is arrogant and cruel, and confident in his abilities.
Eochaid has a fine-boned, cruel face like an eagle, and long bronze hair. He wears a bronze half mask as well as a bronze cloak and armor etched with designs of waves and the sea.
- Longevity: As a Rider of Mannan, Eochaid is very old and has been alive for thousands of years.
- Enhanced strength and speed: Eochaid is able to move incredibly fast and deal heavy blows.
- Deceit: As a Rider of Mannan, Eochaid was wild magic and could lie despite being a faerie.
- Lord of Shadows (first appearance)
Eochaid is an old name of Irish origin, used by many legendary figures.