WARNING: There are SPOILERS ahead for The Expanse season 5.
The shocking ending of The Expanse season 5 finale shows a cryptic transmission from the planet Laconia — along with a sinister-looking object being built in the background. No doubt many viewers were left wondering what the protomolecule is building, and how this will factor into the main story in The Expanse season 6. While these and other questions largely remain a mystery, there are some clues from the scene that suggest what will happen next — and, of course, the source material itself also provides some insight.
The Expanse season 5, episode 10, "Nemesis Games," named for the James S. A. Corey book the season is partly adapted from, had a twist ending: a Martian ship being destroyed as it exited The Ring. No explanation was provided for why it happened, and the scene itself was vague enough that it is still open for interpretation; however, the novel Nemesis Games is more clear that the Barkeith and its crew are obliterated by an unknown anomaly. Based on visual similarities between The Expanse season 5's final scene and the effect used for the Unknown Aggressors in the season 4 finale, it looks as though Holden's worst fears may have been realized: The alien force that killed the Ring Builders is aware of humanity's presence in the universe, and they are indeed angry.
Just before the Barkeith is transformed into a swirling red cloud, Admiral Sauveterre and Lt. Babbage were shown receiving a transmission from Paolo Cortázar on planet Laconia. Cortázar is the protomolecule researcher who was taken by Anderson Dawes from Tycho station in season 2. He indicates that his side has received the "sample," that their "construction" on the structure in the sky is almost ready to enter the next phase, and that the structure's inner workings share similarities to those observed on Ilus. Cortázar thus sets up new villains for The Expanse season 6; in addition to Marco Inaros, the main characters will have to contend with another enemy — one that is using Ring Builder technology discovered on the planet Laconia.
The Expanse season 5 gradually revealed that a group of Martians had been supplying Belter terrorist Marco Inaros with advanced combat technology, which he was using to attack Earth. While the motivation initially seemed to be financial in nature, eventually Bobbie Draper, with help from Alex Kamal (who won't be returning for The Expanse season 6), discovered that even high-ranking officers like Admiral Sauveterre and Lt. Babbage were involved, pointing to a conspiracy rather than purely greed-driven organized crime. The season 5 finale confirmed not only that the protomolecule was being traded for payment, but also revealed why the dissenting Martian officers wanted it: because they had discovered ancient advanced technology on the planet Laconia, and they need the protomolecule to activate it, just as the Rocinante crew inadvertently triggered the technology on Ilus by bringing traces of the protomolecule to the planet.
Cortázar's short transmission also indicates that the protomolecule could be used to build structures, just as it built the Ring. It appears that he and his Martian team have discovered a way to update their own technology using whatever they discovered on Laconia, and perhaps have discovered a way to better control the protomolecule. This follows the events of the books, in which a breakaway Martian Congressional Republic Navy (MCRN) fleet discreetly organizes a secession from the MCR in order to establish a new nation on the planet Laconia, where a probe discovered a Ring Builder shipyard. The advanced technology allows them to update their own ships, giving them a significant technological edge over the rest of humanity. Eventually, the nation emerges as the powerful and despotic Laconian Empire. With The Expanse season 6 being the show's final season, it's currently unclear how the Amazon Prime show will wrap up the Laconia storyline, which extends beyond the sixth book in the series, Babylon's Ashes. What is clear, however, is that James Holden and his crew are in for quite the battle.