The Cleansing was a ritual practiced by the Tomol until the year 2269.
The ritual was probably instituted by the Preservers when they transplanted the Tomol from their original homeworld to Nereus II/Arethusa. Because all Tomol underwent the Change shortly after reaching adulthood (usually just past their seventeenth sun-turn), an event that carried with it god-like powers but also insanity, Tomol children were taught from birth that they would have to undergo the Cleansing shortly after the first signs of the transformation manifested themselves.
The ritual took place at the Well of Flames, where the villagers would gather around, witnessing the Changed throwing themselves into the fire, accompanied by ritual chants, overseen by the high priestess and guarded by the Warden. (Seekers novels: Second Nature, Point of Divergence)