In the final scene of The Expanse season 5, the Martian ship exits the Ring and appears to be frozen in time — before being wiped out by an angry red force. Although the moment is unclear, the source material Nemesis Games sheds some light on what happened to the rogue Martian vessel, the Barkeith, which was wiped out of existence as it exited the Ring to approach the planet Laconia. The moment is more than an exciting, dramatic twist: The final, chilling scene is a reminder of the largely-unknown threat that still lurks around the Ring, setting up a dramatic conclusion for The Expanse season 6.
The Expanse season 5 ended with the climax of Marco Inaros' plan: having delivered the protomolecule to a defector faction from Mars, the Free Navy and their new allies are able to overwhelm the Martian warships guarding the Ring. Admiral Sauveterre and Lt. Babbage are confirmed to have been collaborators in Marco's terrorist plot and are shown exiting the Ring headed for the planet Laconia, where other rogue Martians are building a mysterious ship using Ring Builder technology.
The precise moment the Barkeith entered the Laconia ring, the ship was frozen in time and became translucent. The ship and all its crew were then wiped out by a mysterious cloud-like form, their very particles being seemingly erased. Although it's not explicitly stated in the episode, the Barkeith was destroyed by the Unknown Aggressors: the beings that wiped out the Ring Builders previously, and the likely villains of The Expanse season 6.
The Unknown Aggressors are aliens introduced in The Expanse season 4. After crossing through the Ring Gate and landing on the alien world Ilus, Holden learned that the aliens responsible for the protomolecule, dubbed the "Ring Builders," were once an advanced civilization that was wiped out by an unknown force. After the protomolecule remnants restarted ancient devices on Ilus that threatened the lives of the new settlers, Holden, The Investigator, and Dr. Elvi Okoye located and used a remaining weapon left behind by the Unknown Aggressors: a mysterious sphere that — when The Investigator passed through it — destroyed the remaining protomolecule traces and shut down the planet's devices.
Thus far, only two characters have had an encounter with the Unknown Aggressors. Holden's relationship with The Investigator — a construct created by the protomolecule to discover what happened to the Ring Builders —provided Holden with (limited) insight into the Ring Builder's history. Holden even experiences visions of the Ring Builder's being wiped out in season 3. In The Expanse season 4, Dr. Elvi Okoye had a similar experience when she passed through the sphere in "Cibola Burn." She describes the experience to Holden saying, "I could feel the spaces between things." She states that she felt a "presence" moving towards her — and Holden replies that he feels the same thing too, every time he goes through a ring.
The visual of Elvi going through the sphere in "Cibola Burn" is visually similar to the final shot in The Expanse season 5. Furthermore, her description of the experience matches the description of what happens to the Barkeith in the novel Nemesis Games. The text describes Sauveterre as suddenly being able to see "the spaces between the molecules," and just as he's trying to shout orders, a mysterious force — "something else[...] passing through the spaces between the spaces" — rushes towards him, killing him. No doubt this is what happened at the end of The Expanse season 5, setting up the Unknown Aggressors and the Laconian Empire to be the main focus of the show's final season.