Gehenna - The End Of All Kindred

Gehenna scenarios are fairly well outlined in the Book of Nod, VTDA: The Erciyes Fragments , VTM: Revelations of the Dark Mother and other sources, so much so that Gehenna cultists can provide Jack Van Impe-style outlines of the events leading up to the end of Kindred society. While certain signs (notably the prophecies of Octavio) outline peripheral events, the main sequence from the Book of Nod is roughly as follows:

The Time of Thin Blood. Caitiff and Thin-Blooded Vampires become more common, leading to Caitiff Princes. This particular warning is used by Princes to rationalize offices such as the Scourge, eliminating out high-generation vampires.

Increased violence by Garou and other traditional enemies.

The rise of ancient vampires, notably the Crone, until the Antediluvians themselves rise and "break their fast on the first part of us"
The darkening of the Sun

The return of Caine to pass judgment on all Vampires from Enoch, followed by a duel between the Dark Father (presumably Caine) and the Dark Mother (presumably Lilith).

A thousand year reign from the city of Gehenna, with "no love, no life, no pity/the mighty will be as slaves/the virtuous will be made foul" through the Dark Father's agency.
The arrival of the Last Daughter of Eve who will decide the fate of all creatures.

The Erciyes fragments provides additional commentary and extensions on these events.

Even without Noddism, there is ample reason to believe that Gehenna or something like it will eventually occur. As vampires age, they must sustain themselves on increasingly potent blood. Vampires of low Generation are more powerful than their descendants, and there are various potent Disciplines which have particularly strong effects on an elder's own descendants and other higher Generation vampires (which, good little Camarilla vampires should be by virtue of not staking and draining their ancestors). Given the power gap between vampires of 8th and 7th Generation, imagining a hungry Third Generation blood god isn't a large leap.

Vampiric culture is broadly divided on Gehenna. Officially, the Camarilla doesn't acknowledge Gehenna, Antediluvians, ancient blood gods, or Caine. Conversely, the Sabbat is a Gehenna cult - its raison d'etre is to provide muscle for Caine's return to judge the (un)dead and the living.

While the Camarilla ostensibly observes a "no-Gehenna" policy, it doesn't enforce that particularly heavily - princes will use the Book of Nod to justify decisions, and various clans (notably the Nosferatu) openly prepare to defend themselves against their Antediluvian.

Given this, there are a variety of visible (insofar as a paranoid secret-mongering race of nocturnal monsters makes things "visible") Gehenna cults throughout recorded history. Given that Kindred are on the whole pragmatic, self-interested and secular, these cults can loosely be viewed as making sure that when the Antediluvians come, there will be someone else available to eat. So maybe there isn't a Caine, but if sacrificing a virgin every Walpurgisnacht ensures that grandsire doesn't devour your unliving soul, the world can stand a few less virgins.

Starting in 1999, Gehenna became a pressing concern for many Kindred. With the appearance of the Red Star and the actual rising of Ravnos during the Week of Nightmares, Gehenna stopped being a matter of speculation and more a matter of survival. The Week of Nightmares was a wake-up call for Kindred on both sides of the Gehenna fence.

For the skeptics, Ravnos' rise was disturbingly violent proof for the existence of Antediluvians (or some kind of unimaginably powerful, vampire-devouring blood God, at which point Noahide distinctions are largely academic). These new events led to an upsurge in Gehenna cults and the purchase of heavy weaponry among the Camarilla vampires.

However, for Gehenna cultists, the results were equally disturbing, most notably for the True Black Hand. For centuries, Gehenna cultists had been waiting for the Antediluvians to return. When one does, it barely lasts a week and during that time, didn't pay the slightest attention to them. For true believers, this was a horrifying possibility, not just because they might be wrong, but because it may imply that no matter what happens, they might be lunch.

Another in-game mechanic for Gehenna involved misinterpretation of the Book of Nod. The rationale for destroying the Thin-blooded came from the Book of Nod's prophecy that a rise in high-generation vampires was a sign of Gehenna. However, the actual event was the deaths of large numbers of vampires; since high-generation vampires die easily, they die more often. The constant slaughter of Ravnos in India caused by wars between the Ravnos and the Infinite Thunders Court led directly to Ravnos' rise — a detail unknown to Princes pushing pogroms for the last generations.