The various myths associated with the Klingon gods are obscure prior to the invasion of the Hur'q forces in the 14th century. The sacking of Qo'noS and occupation of the Klingon homeworld had a distinct affect on Klingon culture.

The eventual uprising of the Klingon people killing and driving off the Hur'q invaders became a central narrative in Klingon society. There are references to an earlier myth in which the Klingon gods deemed Qo'noS and the Klingon people to be perfect and left their creation. However, this version is supplanted after the uprising against the Hur'q with the familiar present narrative where the Klingon people kill and overthrow the gods.

The earliest extant telling of the downfall of the gods is in the form of an epic ballad dating from the late 15th century CE attributed to the poet Lusor. However, little is known about the poet apart from the lake which shares his name in the region he was supposed to have lived. Modern scholars (K'vok, Soltan, et al.) have theorized that the work is a collection of various folklore that was standardized and promulgated by the officials of the 16th century warlord Molor in order to cement the legitimacy of his rule by asserting his family ties to those Klingons who, according to this version of the ballad, overthrew the gods.

EDIT: Requested citations:

T'lon Concerning the Resolution of the Conflict between the Klingons and the Hur'q: A New Examination Vulcan Historical Society, 2283

K'vok, son of Valkor, The Klingon Theopolemon - a new English Translation and Commentary, Case Western Reserve University Press, 2274

Soltan Qun maghpu’, University of Andoria Press, 2270

Given the nature of human historiography over similar time periods, Klingon history is heavily mythologized until comparatively late in the post-Hur'q epoch. Sorting out what actually happened isn't helped by the fact that the Klingons have only begun to enjoy relationships with other cultures that allows historians from outside the Klingon tradition access to primary source material. It doesn't help that Klingons are often reluctant to discuss their history and beliefs with Western Federation historians.

That said, we know that Kahless was an historical figure; in spite of the dramatic damage to the material record done by the Hur'q occupation of Qo'noS there is archeological evidence for Kahless, including preserved genetic information.

The Klingon cultural tradition holds that they killed their gods a millenia ago, and there are certain events that suggest that this may be based on actual events, though depending on the depiction it's not totally clear which gods are being killed by whom.

An easy assumption to make is that the gods in question were the Fek'lhri; there exists evidence of conflict between Kahless and a group of that name, and of course the etymology of the term is certainly suggestive of Fek'lhr, the hideous guardian of the Klingon hell, Grethor. I'm prepared to suggest, however, that this more traditional view is mistaken; rather than denoting the literal troops of a devil, I suggest that Fek'lhri is simply a widely-applied epithet.

Instead, I'm prepared to suggest that the god slain was, in fact, Molor. The Klingon wedding ceremony depicts a (romanticized, obviously) version of the destruction of their gods at the hands of Kahless the Unforgettable and Lady Lukara, but their union is most famous not for opposition to the Fek'lhri, but for their uneven battle against Molor and his army. The ceremony itself ends with the culmination of the journey to Kal'Hyah; the ceremony is a Klingon creation myth in which the union of Kahless and Lukara is so powerful that the gods are destroyed, followed by a reenactment of the most romantic battle in Klingon history against Molor's troops.

In this conception, the destruction of the Klingon gods is a depiction of the "creation" of Kahless and Lukara at the hands of the tyranny of Molor, who is then destroyed by what he unleashed. This interpretation has gained some traction recently as it fits a little better with Klingon conceptions of honor and justice, as well as the tradition of challenging and unfit leader, than the traditional view that the gods were just some guys that Kahless killed and/or imprisoned in Grethor.

the Slaughter, Dalg'Roth - Reinterpretations of Klingon Material Culture in the 2nd Empire. Quo'noS University Press, 2355.

McCloskey, James - The Road to Gowron - Toward and Understanding of Klingon Political Trends from the Time of Kahless to the Dominion War. Princeton University Press, 2379.

Wallis, Patrick and Webb, Cliff - A New Face for Kalhless - the Consequences of the Emergence of the Emperor. Klingon History Working Papers, 128/09. Department of Klingon History, Proxima School of History and Political Science, 2280.

When you analyze Klingon history, you need to make sure you do it from a Klingon point of view. When we impose our humano-centric "fact-based" worldview on everything, we are engaging in more of the cultural imperialism for which people throughout the quadrant resent the Federation.

The best answer to your question can be found in the musings of the famed philosopher-warrior, Worf son of Mogh, on the topic of the death of terran culture-hero Davy Crockett. Essentially, both viewpoints are wrong. The basis of your questions is wrong. The only real question is whether your believe in the legend of the first Klingons or not. If you do, there should be no question in your mind that, standing united, the first Klingon man and woman slew their gods and cast them into Gre'thor for all eternity.

As I recall, evidence for such a tale was discovered during the Full Circle project undertaken by the USS Voyager and her support fleet and the multiple so-called "Invasions of the Furies" during 2267 and 2371.

The Full Circle project found evidence of visitation on Qo'nos by an advanced species thousands of years earlier. This visitation resulted in the deliberate genetic development of the "Feklhr" gene in a small portion of the early Klingon population.

Similarly, it was discovered that Qo'nos was also visited by a race named "The Host". This race is believed to have inspired many myths across the universe including the human Devil and the Klingon Feklhr. It is known that the early Klingons fought off these beings.

So, knowing this, one can suppose that this early myth sprouted from one, or perhaps an amalgamation of both, of these early encounters.

seriously wtf is going on with this subreddit? Is it because it's April Fools in Europe?