This Gilded Abyss (This Gilded Abyss #1) by Rebecca Thorne

Sergeant Nix Marr is a damn good soldier. She's also desperate to leave her haunted past deep in the bioluminescent ocean, buried alongside her best friend, Quian. So, when Subarch Kessandra, Valkesh's favorite royal--and Nix's loathed ex--requests Nix's help investigating a masacre in the abyssal city of Fall, Nix refuses. Vehemently.

She should have known Kessandra would fight back.

Consigned as Kessandra's bodyguard, Nix grudgingly boards the Luminosity, a luxurious submersible that offers the only transportation to Fall. But Kessandra wasn't truthful--surprise, surprise--and her "investigation" isn't about the massacre, but rather what caused it: an illness that incites its victims into a violent craze.

When another royal is brutally murdered, Nix and Kess realize the disease has spread--and no one on the Luminosity is safe. If they're going to survive until Fall, they'll have to trust each other... but considering Kessandra is responsible for Quian's death, that won't be easy. Softcover; 446 pages