Thel was born Thel 'Vadam to the Vadam Family, rulers of the powerful State of Vadam on Sanghelios. As a young Sangheili, Thel was trained in the ways of the warrior by his relative Lak 'Vadamee. He joined the Covenant military, earning him the right to use the "-ee" suffix, which is added to a Sangheilian name upon joining the military. Thel fought alongside the Covenant when they attacked Madrigal in 2528.
Soon, Thel gained the rank of Shipmaster and earned the title of Kaidon.
Soon after being promoted to Shipmaster, Thel suffered an injury in the training ring due to a mistake. His family hushed up the incident, as well as the resultant visit by a doctor, due to Sangheili culture equating the loss of blood and medical operation by a doctor with a loss of honor. During his convalescence, Thel handmade an old-fashioned doarmir-fur Shipmaster's cloak, wearing it always to remind him that he could make grave mistakes if he let his guard down.