Having read 'Ghosts of Onyx' a couple times, I'll try to lay out the core plot. It's not just another 'Master Chief shoots aliens' story. The main focus shifts to the SPARTAN-III program, a more mass-produced, 'suicide soldier' initiative created by Colonel James Ackerson as a response to the staggering losses of the earlier SPARTAN-IIs. We follow a company of these younger Spartans, led by Kurt-051 (a Spartan-II who was secretly pulled into the program), as they undergo training and then get thrown into an insanely dangerous mission to destroy a Covenant shipyard.
The 'Onyx' part comes in when the surviving Spartans—Kurt, some S-IIIs, and a handful of Spartan-IIs like Fred, Kelly, and Linda—get stranded on the mysterious Forerunner planet Onyx. The planet itself is a giant construct, a Shield World, and most of the book becomes this desperate fight for survival inside a labyrinth of shifting passages and ancient technology, all while being hunted by Covenant remnants. The ending sets up huge implications for the Halo universe with the discovery of this Shield World and its purpose. It's a much more tragic, ensemble-focused book than some of the others.
The heart of the book for me was always Kurt's story. He's torn between his old Spartan-II family and his new duty to these kids he's training, knowing most are destined to die. The battle on the asteroid to take out the shipyard is brutal and quick, really hammering home the 'disposable' nature of the IIIs. Then, the whole second half on Onyx feels like a completely different genre—a sci-fi survival mystery. Dr. Halsey shows up too, which adds another layer of conflict with Ackerson's legacy. The plot can feel a bit split, but it works because the emotional throughline is Kurt's impossible choices.