A fearsome beast of Klingon mythology said to guard Gre'thor, the underworld of the dishonoured dead, whose shape the trickster Ardra borrows to frighten the Enterprise crew.
Fek'lhr is a creature of Klingon myth, the guardian of Gre'thor, the place to which the dishonoured dead are condemned. Klingons insist that he is not a devil in the human sense, since their tradition has no such figure, but he is the nearest thing their stories offer.
He is seen in The Next Generation episode Devil's Due. The con artist Ardra, posing as the devil of Ventax II, shifts through the demons of several cultures to unsettle Picard's officers and for Worf's benefit takes the form of Fek'lhr, a towering, horned and shaggy beast. Worf identifies the apparition at once, though it is only one of Ardra's technological illusions.
Gre'thor itself is later visited in visions in the Voyager episode Barge of the Dead.