The Ferrets series
From Halopedia, the Halo wiki
The The Ferrets series is the retroactive name given to Halo novels focusing on the Ferrets, written by Troy Denning. It consists of Halo: Last Light (2015), Halo: Retribution (2017), and Halo: Divine Wind (2021).
The series also shares a direct narrative connection with Halo: Shadows of Reach, with its characters and events leading directly into Halo: Divine Wind. It also shares a narrative connection to the short story Sacrifice as the Covenant loyalists appear in Halo: Divine Wind.
Halo: Divine Wind has narrative connections to Halo Wars 2, continuing the story started there. Halo: Retribution has direct narrative connections to Halo 5: Guardians, specifically the events aboard the Argent Moon in the level Blue Team which are also slightly expanded upon in Halo: Divine Wind.
All of the novels in the series feature the Forerunner archeon-class ancilla Intrepid Eye as well as the Covenant remnant the Keepers of the One Freedom and its leader Castor as major recurring antagonists.