One of the current big bads in this season of Supergirl is an organization/syndicate called Leviathan. This mysterious group has several shadow operatives and plans to remove the current human power structures on Earth. Meanwhile, one of the newest villains in current DC comics is also called Leviathan. Much like The Batman Who Laughs, Leviathan has had an undeniable impact on the DC Universe in the past year, heavily affecting numerous titles.
However, despite having the same name, these two entities are quite different. It's actually rather weird to have a concept that is currently receiving such high profile use in the comics get the same on-screen, but in such a different way. Here's a rundown on both versions of Leviathan, as well as the subtle similarities and drastic divergences between the two.
DC Comics' Leviathan
The original incarnation of Leviathan organization appeared in 2011 and was an anti-capitalist offshoot of the League of Assassins led by Talia Al Ghul. The modern incarnation, however, has appeared in the eponymous Event Leviathan limited series. This version's goals are to drastically tip the scales of order and be far more proactive than groups such as the Justice League when it comes to dealing with, and preventing, the world's issues. To this end, the organization has even gone so far as to try to recruit superheroes who are also tired of the status quo.
The leader of Leviathan also bears this name, and his identity is that of a former hero, as well. Leviathan was revealed to be Mark Shaw, who once operated under the mantle of Manhunter. Shaw had joined the Manhunter cult, which derived its name and mission from the alien androids. These robots were a taciturn police force employed by the Guardians of the Universe before being abandoned in favor of the Green Lantern Corps. His new Leviathan organization is an attempt to similarly police the planet Earth, as Shaw has grown tired of both the burden being solely on superheroes, as well as their reactive methods. To this end, Shaw has enlisted metahumans, as well as several other operatives across the globe in an attempt to shatter the status quo of security, putting the fate of the world back in its peoples' hands. To possibly safeguard against those who wouldn't go along with the organization, Shaw even employed technology capable of taking on some of the biggest DC heroes.
The Leviathan on Supergirl is similarly a shadowy organization with eyes and operatives everywhere. Finally revealed in the recent episode "Tremors," this Leviathan is led by a very ancient alien. The members of Leviathan trace their origin to Jarhanpur, a sister planet to Krypton. Fleeing the planet before its civil war led to its destruction, they came to Earth and hid for millennia. Their leader, Rama Khan, has the power to control the Earth and its natural forces and was behind several "natural" disasters throughout human history. Along with Gamemnae, Rama Khan intends to forcibly take Earth back from humanity, who they see as a scourge to it. Rama Khan and Gamemnae first appeared in Justice League comics of the early 2000s, where they and Jarhanpur instead had terrestrial origins tied to magic. The idea of them operating since early human history was retained for the show versions, however.
Differences and Similarities
It would seem, at first glance, that the only similarities between these two Leviathans would be nominal in nature. There are some rather subtle things that tie the concepts together, however. Besides the name, both Leviathans are mysterious, far-reaching organizations that have operatives every which way. They also intend to dramatically change the power structure of Earth, though in staunchly opposite ways. The former seeks to put humanity's fate back in the hands of the people and off of the shoulders of superheroes, whereas the latter wants to give Earth to the aliens of Jarhanpur and out of humanity's clutches. That Leviathan in the comics has also targeted Superman, in particular, gives both versions a loose connection to Kryptonians. Both versions also repurpose previous concepts from the comics, with the former revitalizing an idea from right before the New 52 and the latter by reimagining Post-Crisis Justice League villains.
Even these connections are fairly minimal, and it's rather uncanny that the name is being used in such different ways in the same timespan. It might have been intentional to "adapt" the current villain organization in this season of Supergirl, but the lack of any real connection says otherwise.