|“||We are the Riders of Mannan! We have slain the Firbolg! We have no fear of giants!||”|
–Ethna's last words, Queen of Air and Darkness
Ethna was a powerful faerie apparently born to Mannan and had lived almost since the beginning of time. In the old days, she hunted giants and monsters with her brothers, but eventually they started serving the Unseelie King as his personal hunters and assassins. At one point, the Hunters were dispatched by the King to hunt down and kill his daughter, Auraline, the First Heir, and her eventual offsprings.
In September 2012, the King sent the Riders after the Blackthorn family to retrieve the Black Volume of the Dead. Ethna went after Emma Carstairs and Julian Blackthorn in Cornwall, along with Delan, Airmed and Fal.
Alongside her siblings, Ethna fought Emma until she killed Fal with her sword Cortana. Devastated by Fal's death, Ethna went ballistic and had to be held back by Delan. When Airmed called their steeds and they retreated, Ethna paused to glare at Emma in a way that promised revenge.
She later attacked the Blackthorns in London, using a human child to lure them out of the Institute. They almost succeeded in killing the Blackthorns until Annabel Blackthorn arrived and drove them away by threatening to reveal the Unseelie King's name.
Upon returning to the Unseelie Tower, Ethna and her brothers were assigned to guard the King's prisoner, Ash Morgenstern. She and Eochaid caught Julian and Emma in Ash’s room trying to rescue him and took them to the prisons, for they were in the tower and she could not kill them without the King’s permission. There, the Shadowhunters joined Clary Fairchild and Jace Herondale who had also been caught by the Riders at some earlier point.
During the fight in the throne room between the Unseelie King, the Seelie Queen, and the prisoners, she and Eochaid tried to defend the King but failed. Ethna was upset by the King's death and fought Emma, almost killing her, before she was tackled and repeatedly stabbed by Julian with an iron figurine Simon Lewis had given him. Both Shadowhunters fled to Thule in time to escape her and her brother.
Shortly after, Ethna and her siblings went after Kit Herondale during the Cohort's parley at the Imperishable Fields, intending to murder the boy for being a descendant of the First Heir. When Ethna told him that they had killed his mother, Kit uncontrollably attacked them with magic, making the Riders' horses disappear in the air when they tried to flee.
They fell onto the battlefield where they engaged in the fight against the Shadowhunters and their Downworlder allies, determined to complete the mission their late King had given them—murdering the Blackthorns and their allies, thus choosing to be considered traitors to the new Unseelie King, Kieran. They had the upper hand in the fight until Hypatia Vex appeared and temporarily dispatched them with her magic.
When Emma and Julian's parabatai curse was activated during the battle and they turned into true Nephilim, Emma challenged the Riders. Fearless, Ethna was the first to lunge at giant Emma, who tore her apart, killing her on the battlefield.
Ethna was bloodthirsty and enjoyed seeing the suffering of their opponents, showing no mercy. She was very passionate about her loyalties and duties. She appeared to care for her siblings very much and showed the most anguish when one of them was hurt. She was also very vengeful and cruel and not above using children to complete her missions.
Ethna was of slighter build than the other Riders. She had metallic bronze hair in a braid down her back and wore a bronze half mask, as well as a bronze cloak and armor etched all over with designs of waves and the sea. Her longsword was sometimes slung over her back, seen over her shoulder.
- Longevity: As a Rider of Mannan, Ethna was very old and had been alive for thousands of years.
- Enhanced strength and speed: Ethna was able to move incredibly fast and deal heavy blows.
- Deceit: As a Rider of Mannan, Ethna was wild magic and could lie despite being a faerie.
- Ethna had the bronze armor, mask, gear, and weapons, including a longsword, that all the Riders possessed, as well as a steed that coalesced from the air when summoned.
Ethna is an Irish name that means "graceful".
- Ethna was the only female Rider.