From Halopedia, the Halo wiki
A skein, also described as a master-pack, is a major social division in Jiralhanae society, formed out of multiple clans sharing a similar philosophy and common interests. Described as "meta-clans", skeins are generally dominated by the most powerful clans. These skeins are profoundly different in culture, philosophy, and their pursuit of power. The two most powerful skeins are the Rh'tol and the Vheiloth; the Rh'tol are arguably more primitive, while the Vheiloth are more sophisticated.
The First Immolation was initiated by Chieftain Rukt of the Rh'tol skein, when a protracted territorial dispute led to a violent clash between the rival Rh'tol and Vheiloth skeins. The rival master-packs waged widespread mechanized warfare against each other across the surface of Doisac, causing such destruction that caused the collapse of Jiralhanae society and reduced their species to a pre-industrial state. When the Covenant later arrived at Doisac, the skeins fought against the alien empire. While the Vheiloth skein adopted Covenant religion without challenge, the Rh'tol only did so for strictly practical reasons. Tensions between the Jiralhanae skeins were controlled by Covenant leadership, though cultural friction and competitive infighting between master-packs remained common.
During the Human-Covenant War, the young Tartarus climbed to power and had begun to gain authority throughout the entire Rh'tol skein. Catching the eye of the High Prophet of Truth, Tartarus would work with the Hierarch throughout the war as Truth intended to replace the Sangheili with the Jiralhanae. Late in the conflict, Truth elevated Tartarus to Chieftain of the Jiralhanae, giving him complete control over the Jiralhanae, regardless of skein. When the Covenant collapsed in the Great Schism, the Rh'tol skein gloried in the piracy against former client species of the empire, while the Vheiloth skein saw looting and destruction as an unfortunate necessity. Lydus was the chieftain of a master-pack based in the Oth Sonin system.
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