The Endorian Holocaust is the theorized devastation of the fictional forest moon of Endor after the destruction of the second Death Star (in Star Wars Episode VI: Return of the Jedi) by the impact of falling debris. It has not been established to have occurred in any canon sources, but rather has been proposed by individuals in fandom as a scientifically inevitable consequence of the events portrayed.
The second Death Star was a sphere of machinery somewhere between 180 and 900 kilometers in diameter (depending on the sources one holds to be authoritative) and it was located very close to Endor's forest moon when it exploded. Furthermore, given its close proximity and the moon's relatively Earthlike gravity, it was likely that its geostationary orbit was being artificially maintained. Much of the debris from the explosion would have subsequently rained down on the moon, producing a meteor storm of titanic proportions. The relative tranquility of the region immediately around the former shield generator in the brief celebratory scenes set there after the Death Star's destruction has been attributed to active deflection of debris from this area by Rebel forces, but the Holocaust's ecological effects are predicted to be drastic enough that this would provide only temporary respite even there.
If the Endor Holocaust occurred, it would certainly have led to the extinction of Ewoks on the forest moon of Endor. However, it has been established in Star Wars comics and novels that some Ewoks had been removed from Endor in the past for use as pets or slaves, so it is possible that the species itself survived even if the holocaust occurred as predicted. It should also be noted that the Return of the Jedi novelization has stated that the Emperor had issued orders that if the Rebel strike force on Endor's moon successfully disabled the shield generator the Death Star should immediately destroy the moon itself, so once the shield generator was disabled (with significant Ewok assistance) the fate of Endor was likely sealed.
Some Expanded Universe sources maintain that Endor was not significantly affected by the destruction of the Death Star and life continues there as normal. One other suggested it was destroyed, but the author had taken pains to paint it as Imperial propaganda, and thus supposedly unreliable, despite it being the scientifically mandated result. Pro-Holocaust debaters speculate that the well-established pro-New Republic bias in most of the literature was responsible for the apparent coverup. Another rationalization involves a rapid and strenuous effort so the planet can regain a semblance of normalcy for its later appearances, though it is not clear where the supposedly resource-strapped Rebel Alliance will find the tools to do so.
In one of the Star Wars Tales comics, an Imperial veteran of Endor makes an apparent reference to the holocaust theory, after telling the story of his unit's trouble with the Ewoks in a bar. Another character dismisses it as a myth, saying that most of the Death Star's mass was obliterated in the explosion, and that the Rebels "took care of the rest."
In the 'Jedi Academy Trilogy', by Kevin J. Anderson, the character of Kyp Duron visits the site of Darth Vader's funeral pyre on Endor. There is no indication given in the planet has suffered any cataclysm.
"Inside the Worlds of Star Wars Trilogy" describes how the Rebels managed to use shields and tractor beams to protect their strike team on Endor, but by possible implication (supported by the title of "The Aftermath of Victory") dooming the rest of the forest moon to the ecological disaster.
The two Ewok telefilms are set on a non-devastated Endor, but it is unclear whether the events depicted occur some time before or after Return of the Jedi.
The core of this argument is one between the inevitable consequences of G-canon versus C-canon. The Pro-Holocaust people use the position of G-canon, and without a mechanism, C-canon cannot override G-canon in direct contradiction. The anti-Holocaust insist that C-canon can override G-canon just because the film didn't show the debris actually hitting Endor, and that it would be uncharacteristic for the Rebel characters to callously celebrate amid a friendly population doomed by their actions.
Some also argue "author's intent", that Lucas did not intend for Endor to die. The pro-Holocaust argument holds that under suspension of disbelief, author's intent did not count, under the assumption that even a non-scientist like Lucas can hardly have thought blowing up his 900km sphere right next to Endor in a manner that clearly produced a fragmentation pattern could possibly be good for it.
The debate continues among die-hard Star Wars fans on the internet, sporadically.