Zerthimon

Zerthimon is a mytho-historical character in the Planescape setting of Dungeons & Dragons, revered by the githzerai as their spiritual leader and founder, who fought alongside Gith to free them from the illithids and then spoke up against her tyrannous plans, causing The Sundering Of Two Skies that divided the ancient gith-kin into the modern githzerai and githyanki.

Unusually, Zerthimon's status and to an extent his lore has actually changed in between edition.

When the githzerai were first detailed in the Outer Planes Appendix for the Monstrous Compendium, Zerthimon was actually a mythical figure, said to have perished in battle against Gith and from there ascended to a demigod status. It was believed he was building his strength elsewhere in the planes, and would ultimately return to lead the githzerai to true paradise. This meant that the githzerai Zerths were not only multiclassed fighter/mages, but also acted as priests of Zerthimon. This strange religion was an underground phenomena, as Zaerith Menyar-Ag-Gith, wizard-king of the githzerai and their equivalent to Vlaakith CLVII, saw it as a potential threat to his authority.

Fate was to intervene when Black Isle created the CRPG Planescape: Torment. In it, they introduced the party member Dak'kon, stoic and grim Zerth, whose crisis of faith lead to the sacking of Shra'kt'lor by the githyanki. He presented Zerthimon not as a myth, but as a matter of fact, and removed all mention of both "The Great Githzerai" and Zerthimon's supposed death & ascension. Dak'kon's huge popularity went on to solidify Zerthimon's presence from mere religious belief to a full-fledged fact of the githzerai existence, especially as the githzerai became more atheistic as editions passed.

4th edition, of course, stuck its oar in, with an extensive writeup of the githzerai in "Secrets of the Plane Below". Whilst making no mention of the Unbroken Circle of Zerthimon, which outraged many Planescape grognards, it asserted that Zerthimon was a real figure, who opposed Gith for her ineptitude as treating the gith as anything other than soldiers in her army. They fought, he defeated her, and then led to the githzerai to found their own civilization in the Elemental Chaos. It also presented three common beliefs as to what actually happened to Zerthimon; that he ascended to become a psionic entity analoguous to a god; that he ultimately died, was buried with honors, but then they forgot where; and, interestingly, a new option: that he became a lich to try and guide his people for all eternity, only to flee from them into the depths of the Chaos when he realized what a monster he had become. Amusingly, in-universe, this last option is widely regarded as heresy by githzerai and githyanki, with the latter objecting to the idea that their race's greatest enemy could become immortal, like their own beloved leader.

The Unbroken Circle of Zerthimon

Seeing as how Dak'kon is a Zerth, and thusly effectively a priest of Zerthimon, Planescape: Torment introduced us to the priestly teachings of Zerthimon. Known as the Unbroken Circle of Zerthimon, this is a series of historical lessons and parables that the player can unlock in the game if their Intelligence and Wisdom is sufficiently high.

Befitting Planescape, the Unbroken Circle is a highly contentious bit of lore - not because of its quality, but because at the game's end, if you talk to the Practical Incarnation of the Nameless One, he confesses to having made the Unbroken Circle for Dak'kon. Admittedly, the term is used both for the teachings of Zerthimon and the ever-shifting ring of stone/metal that they're written on. Further, Dak'kon himself refers to the 6th Circle being the one that led to his crisis of faith and the devastation of Shra'kt'lor, and would surely be aware of if it was an outright fake or not, leaving to Planescape fans endlessly debating how much these parables are the "real" ones or not.

But, in the absence of anything else to contradict them, most fans are content to take the Unbroken Circle's parables as factual, and just assume the Practical Incarnation made the physical Unbroken Circle as a token to try and use Dak'kon.