Zelashiel, also known as Zelashiel the Blasphemer, was a Force-sensitive being who utilized the dark side of the Force. He possessed skill in Force techniques, specifically the ability to drain an individual's essence, a subtle art that proved exceptionally challenging for use during combat. The teachings of Zelashiel were ultimately recorded within the Sith Holocron belonging to the Dark Lord of the Sith Darth Revan around the time of the Jedi Civil War.
Sometime in the period spanning from 1000 BBY to 980 BBY, Zelashiel's teachings received commentary within an essay penned by the Sith Lord Darth Bane, who had previously studied Revan's holocron prior to its destruction. Many years afterward, Bane's essay, including his observations on Zelashiel, found its way into the Book of Sith, a collection of historical writings compiled into a single volume by Darth Sidious.
The character Zelashiel was initially brought to light in the resource book Book of Sith: Secrets from the Dark Side, authored by Daniel Wallace. In the third set of endnotes pertaining to the book, Wallace revealed that the character's name was intentionally crafted to evoke the sound of a theophoric Hebrew name. Wallace further explained that this naming choice was meant to give readers the impression that the character had undergone a significant fall from grace. The author also suggested that Zelashiel may have existed before the Sith King Adas or may simply be a figure of myth.