- "This is a very smug state. I never enjoyed visiting here."
- — Forze 'Mdama, upon arriving at Ontom
Ontom is a very old and wealthy state on Sanghelios. The state is the home of the Servants of Abiding Truth's headquarters, a Forerunner structure known as the Temple of the Abiding Truth, which the Servants were allowed to keep after the alliance with the San'Shyuum and the formation of the Covenant. The state's architecture is a blend of pre-Covenant and modern-era buildings. Jul and Forze 'Mdama visited this state when they decided to join the Servants in their campaign against Arbiter Thel 'Vadam.
In March 2553, Ontom was attacked by a force of Jiralhanae insurgents. A bomb was detonated near the Servants of Abiding Truth's headquarters, killing several Sangheili and separating Evan Phillips, a human xenoanthropologist on a mission to Sanghelios, from the rest of his team. A short time after the first bombing, a group of Jiralhanae attacked again but were quickly put down by the city militia. A Jiralhanae from this attack was captured and killed by a mob, but soon afterward a third attack commenced.