The Blooding Years is the Sangheili term for the civil war that raged for five years on Sanghelios and throughout Sangheili-occupied territories following the end of the Human-Covenant War and disappearance of the San’Shyuum. Having operated primarily as warriors in the Covenant hierarchy for thousands of years, the Sangheili were left almost entirely without leadership in the wake of the Great Schism. The power vacuum that followed provided opportunity for those seeking power to try and unite the Sangheili under their banner, and many succeeded, including Arbiter Thel ‘Vadam, Jul ‘Mdama, and Avu Med ‘Telcam.
Initial fighting was fierce, with ‘Vadam’s Swords of Sanghelios defending themselves against ‘Telcam’s Servants of the Abiding Truth, who thought ‘Vadam’s truce with humans was sacrilege. Though the Servants failed and were forced to retreat following the intervention of the UNSC Infinity, the conflict spawned the next major belligerent, ‘Mdama and his new Covenant. ‘Mdama’s Covenant was far more successful than the Servants, growing into a serious threat to Sangheili and regional stability during the mid to late 2550s. The power ‘Mdama was able to amass was nearly sufficient to topple the ruling Swords of Sanghelios, though again with the assistance of the UNSC and the Infinity, ‘Vadam was able to keep power and eliminate ‘Mdama and all remaining Covenant forces on Sanghelios.
These events are depicted in the novel Halo: The Thursday War, the game Halo 5: Guardians which was the subject of a Spoilercast on November 9th, 2015, and the comic Knight Takes Bishop from the anthology Tales from Slipspace, which was the subject of a Halo Book Club on January 3rd, 2019.