The characters of Supergirl appear to have experienced a bout of collective amnesia - with many of them seemingly forgetting about season 5's main villains: Leviathan. Early in Supergirl season 5, the threat of Leviathan was introduced. On the pages of DC Comics, the group is an offshoot of the League of Assassins created and led by Talia al Ghul. Though both were repeatedly featured in Arrow, the Leviathan group of Supergirl is entirely a new iteration.
Originating on Jarhanpur, a sister planet of Krypton, the members of Leviathan were revealed to have arrived on Earth millions of years ago, subsequently wiping out the dinosaurs. So far, only two prominent heads of the organization have been revealed on Supergirl - with The X-Files' Mitch Pileggi playing Rama Khan and Stranger Things' Cara Buono playing Gamemnae. Deeming themselves protectors of the Earth, Leviathan was responsible for a number of seemingly natural disasters across the centuries. Once again believing humanity to be a threat to the planet, the first half of Supergirl season 5 saw Rama Khan attempt to cull the population with a great flood and then the eruption of a supervolcano.
The plots of Leviathan were immediately put on the backburner by the Crisis on Infinite Earths crossover event. Unfortunately, in the wake of the game-changing storyline, Supergirl has yet to truly recover its previously established groove. Unlike The Flash, which split its current season into two distinct pre-Crisis and post-Crisis chapters, Supergirl season 5 has attempted to continue its previous threads amid a new status quo. A major component of the changes was the creation of Earth-Prime and a new central timeline that the shows of the Arrowverse have each found themselves sharing. As a result, everything that happened between Leviathan and the heroes of Supergirl exists in the memories of the core team. This should effectively give Supergirl and her allies a significant advantage over the threat, with Leviathan erroneously believing they remain undetected in the shadows. However, it appears to have done the exact opposite.
Since Crisis on Infinite Earths, Kara has been largely dealing with threats-of-the-week and a fledgling relationship with William Dey. The only ongoing storyline she has actively referenced in many episodes is that Lena Luthor teaming up with her brother Lex. The same can be said for Nia Nal and Alex Danvers. The former recently became embroiled in a deeply personal quest to protect her trans community. The latter, meanwhile, has been consumed with existential angst since quitting the DEO and the death of her father. Now working with the equally amnesiac J'onn J'onzz, Alex has mostly investigated one-off cases or retreated into Andrea Rojas' new virtual reality system. The name Leviathan has long been one they've neglected to mention, much less investigate.
In what could be described as a symptom of how Supergirl is sidelining its own core female cast, the only two characters that have consistently sought to do anything about the threat of Leviathan are Brainy and Lex Luthor. Since being advised to by a doppelganger, Brainy has been working with Lex in secret. Despite that, even they have made surprisingly little progression beyond maintaining that secrecy and Lex trying to ingratiate himself with Gamemnae. Though Leviathan is occasionally revealed as still present and planning something on the fringes of certain episodes, there's no clarity, no substantial developments, and even less proactiveness from the heroes. With potentially fewer Supergirl episodes remaining than planned, there is an undeniable sense of plot stagnation. A seismic shift may be needed if the show wants to get back to the overall promise Leviathan offered in earlier Supergirl season 5 episodes.