Warning: SPOILERS ahead for Supergirl, season 5, episode 7, "Tremors".
Supergirl revealed that the mysterious group known as Leviathan was responsible for killing off the dinosaurs on Earth-38 in the Arrowverse. The episode also revealed the organization's alien origins and their strange connection to a sister planet of Krypton.
When the name of Leviathan was first dropped in Supergirl's season 4 finale, it was something of a shock to comics fans. While the terrorist organization came to be a major part of the Superman family of comics' storyline over the past year, very little had been firmly established about the group beyond their name and their extensive connections. Indeed, it was not until this past week that the origins of Leviathan in the comics were explained and the leader of Leviathan's true identity was revealed.
The Leviathan group presented in Supergirl season 5, episode 7, "Tremors" is quite different, being established as an alien organization with a history spanning back beyond the early days of Man. This is explained in an early scene, which introduces the character of Rama Khan; one of five aliens from the planet Jarhanpur, who traveled to Earth thousands of years earlier. While chastising an underling, Rama Khan speaks of how his people have been guarding the Earth "since our ship, as big as an asteroid, wiped out the dinosaurs."
The brief scene explains something of Leviathan's organization and goals, as we see Rama Khan debate with Gamemnae, another member of the organization. Khan seems to prefer a fire-and-brimstone approach to managing humanity, arranging great disasters that decimate major population centers, claiming that "the rising of oceans and the raging of rivers will always work." Later in the episode, Brainiac-5 identifies Rama Khan as being responsible for several famous historical disasters, such as the volcanic eruption at Pompeii, the Yellow River flood of 1887, the Bhola Cyclone and the Antioch Earthquake. By contrast, Gamemnae seems to prefer the carrot to the stick and offers the use of her technology to Rama Khan for a peaceful solution.
Strangely enough, the characters of Rama Khan and Gamemnae are taken directly from the comics, but they are not entirely similar to the characters we see in "Tremors." The two villains were originally part of a council known as the League of Ancients, who were a sort of proto-Justice League made up of heroes formed in the year 1020 BC. The League of Ancients, however, was made up of magicians rather than aliens.
Regardless, the revelation that Leviathan is made up of aliens secretly ruling the world does explain many of the mysteries regarding the group. Chief among theses is why they sought to manipulate Lex Luthor and Lena Luthor through Eve Teschmacher rather than recruit them directly. Ignoring the classic Luthor stubbornness, there is no way either of them would bow to any group of alien overlords, even if it did give them a chance to destroy Supergirl. It remains to be seen, however, if the events of this season will see the Kryptonians go the way of the dinosaurs.