While he's considered less of a deity and more of a boogeyman, Z'rsar is the third ascended shadowspawn of Sh'ther Kei-Sihr, from back when he was still considered "The Shadow", during the War of Shadows. When his siblings Tahvihsa and Prethao developed more constructive, benevolent powers, which led to their defection to the side of Rencal Kei-Rhai and the other deities of Illiad, Z'rsar's shadowy powers of corruption only became more potent and dangerous. Things eventually culminated in him absorbing so much life energy from the Karrin of Illiad, which he gained through countless atrocities, so that he had access to all Ena-Types, and became the strongest general of the armies of shadow. It is said that he was personally responsible for tainting Valie, Grand Countess of Change.
After The Shadow was absorbed by Rencal, and the War of Shadows was over, Z'rsar disappeared and was never heard of again. Some say that with The Shadow's end, Z'rsar decided to end himself, too. Others say that he keeps scheming from the shadows, always ready to restart his path of pain, destruction and chaos, should the gods ever become complacent. Soon, he became a boogieman, a figure that parents would tell stories about to their kids, so that they would behave.
Modern scholars are divided on matters regarding Z'rsar. Some see him as an unrepentant monster, others see him as a tragic figure who didn't have a choice but be destructive, due to the inherently harmful nature of his corruptive powers, in direct contrast to his siblings' much more benign, constructive powers, but probably would've loved to join his siblings. Nobody is quite sure of the truth, but a holy text called "The Shadow-Duke's Lament", at least implies that Sh'ther Kei-Sihr is saddened that he didn't join him or his other children in The Obsidian Cathedral, and hopes that he, someday, may turn over another leaf, so that they meet again.
Z'rsar isn't really worshiped as a deity on Illiad, and thus he has no tenets to follow.