The Fek'Ihri were a humanoid warp capable species originally native to Qo'noS. The true nature of the Fek'Ihri was a mystery. In the 25th century they were associated in the Fek'Ihri Horde and maintained a fleet of starships. The extra-dimensional Gre'thor served as their base of operations. Fek'Ihri technology was Hur'q-based. (STO mission: "The Gates of Gre'thor")
- The name of the group would infer a relationship to Fek'lhr.
Fek'Ihri had forehead ridges similar to those of the Klingons but more pronounced, extending further towards the top of the head, usually ending in thick thorns. Their eyes were completely dark, their teeth thin and sharp and they had no lips. Fek'Ihri had scaly skin, ranging in color from green to light pink. Hair was uniformly black, and greasy among males. (STO mission: "Blood of the Empire")
The so-called tortured souls had neither hair nor legs, their body ending a purple mist. They also had black skin and hovered above the ground. Tortured souls also survived in space and where employed as fighters in space combat. (STO mission: "Afterlife")
Male masters were muscular and of enormous size, possessing great stamina. The largest known Fek'Ihr was Fek'lhr, three times as tall as other humanoids. His ax itself was larger than a Klingon warrior. (STO mission: "The Gates of Gre'thor")
The Fek'Ihri had two sexes. Female Fek'Ihri, called "ravagers", had a typically humanoid stature, standing approximately 2 meters tall. Male Fek'Ihri were more diversified, there were three basic types: diminutive hordlings, massive masters and the hovering tortured souls.
The Fek'Ihri had an organized military, with enslaved hordlings as the lowest rank and slave masters as equivalents to commanders. They were fierce enemies of the Klingons, intend on conquering the Klingon Empire. Among them were a number of powerful men in charge, though who of them was the supreme commander of the Fek'Ihri Horde is unknown: Herron was styled 'Lord of the Dead' and master of Gre'thor, Molor aspired to become regent of the Klingon Empire and with the death of Fek'lhr, who's lair was hidden in Boreth's underground, the Fek'Ihri invasion ended. (STO missions: "Afterlife", "The Gates of Gre'thor")
In battle, no distinction was made between male and female Fek'Ihri. The Horde's history appeared to be intertwined with ancient Klingon mythology. Until 2409, they were thought to be a myth. (STO mission: "Blood of the Empire")
In 2409, three Fek'Ihri portals opened in the asteroid ring of the Norgh system. Several squadrons of Fer'Jai-class frigates destroyed a Klingon Defense Force battlegroup but were defeated by reinforcements sent to investitage the loss of ships. Shortly after, more portals opened above Qo'noS and the Fek'Ihri Horde started another attack in the name of Molor. (STO mission: "Blood of the Empire")
The same KDF vessel, with Emperor Kahless aboard, arrived in the Boreth system only to find the orbit of Boreth occupied by several Fek'Ihri Horde starships, including a Kar'Fi-class battle carrier. The Klingons defeated the Fek'Ihri ships and cleared the Boreth Monastery from Fek'Ihri ground troops. Through a meditation at the shrine of Kahless, the KDF away team and the Emperor travelled to the Barge of the Dead, where they motivated the dishonored spirits of Klingon warriors and the ferryman, Kortar, to fight against Herron, Lord of the Dead. After he was killed, the Barge delivered the away team to the dock of Gre'thor. (STO mission: "Afterlife")
Emperor Kahless and the away team led the Klingon spirits into battle against the Fek'Ihri defenders of the gate. Upon entering Gre'thor's interior, they fought three semi-translucent Klingon spectres: Cowardice, Treachery and Dishonor. Following an investigation of Gre'thor, the away team encountered, fought and defeated Molor, who had aspired to begin a 1,000 year reign of terror as the new Emperor of the Klingon Empire. Afterwards, Kahless and the KDF crew returned to Boreth, where they found the lair of Fek'lhr. The Klingons killed him, too, upon which the Fek'Ihri portals closed and their ships returned to their realm. The KDF also discovered Hur'q technology among Fek'lhr's remains. (STO mission: "The Gates of Gre'thor").
During the entire encounter neither Chancellor J'mpok nor the Klingon High Council were sure whether the Fek'Ihri were a genuine, extra-dimensional invasion force or a technological feat styled as a mythological enemy. (STO missions: "Blood of the Empire", "The Gates of Gre'thor")