Fek'lhr

From Memory Alpha, the free Star Trek reference

Fek'lhr (veqlargh in Klingonese) is a figure in Klingon mythology, a humanoid beast, resembling a feral, monstrous Klingon, that guards the gates of Gre'thor.

In 2367, a con artist claiming herself to be Ardra briefly appeared as Fek'lhr, using holographic technology. (TNG: "Devil's Due")

The IKS Fek'lhr was named after Fek'lhr. (DS9: "Image in the Sand", "The Siege of AR-558")

When B'Elanna Torres entered Gre'thor, she was surprised to not see a pit of dispair and Fek'lhr torturing the dishonored. (VOY: "Barge of the Dead")

[edit] Background

The Fek'lhr seen in Devil's Due was portrayed by Tom Magee.

Fek'lhr is not considered a deity, because Klingon legend states that they killed their gods. (DS9: "Homefront") Nor is he considered a devil as Kang said Klingons do not have a devil. (TOS: "Day of the Dove")

It is unknown if there is an etymological or mythological relationship between Fek'lhr and the Fek'Ihri, an ancient enemy of Kahless, mentioned by Worf in "The Sword of Kahless".

In Star Trek: Armada, Star Trek: Armada II and Star Trek: Starfleet Command III, Fek'lhr is a class of Klingon starship.