The Ritual of Tellann was performed by the Imass Bonecaster, Olar Ethil, transforming most of the Imass people into the T'lan Imass. It was "the binding of the Tellann Warren to each and every Imass. The ritual made them immortal [...] The life force of an entire people was bound in the name of a holy war destined to last for millennia." The purpose of the ritual was to make the T'lan Imass capable of exterminating the Jaghut people. Kilava Onass was the only Imass known to have escaped the ritual. The Imass also included their hunting partners, the ay in this ritual, creating the T'lan Ay.
Though most T'lan Imass were under the impression that there was only one Ritual of Tellann, the ritual was again performed later in time by the Brold Imass, descendants of some of those who sought to follow Kilava Onass when she rejected the first Ritual. According to Nom Kala, the second ritual took place about a hundred generations after the Jhagut had gone and about two hundred generations after the K'Chain Che'Malle had journeyed to the eastern lands.