He was the inventor of the Ark of Truth. It was not, however, used in his lifetime. The other Alterans ruled that they could not use it because it would deny the Ori free will. He was forced to leave it behind at Ortus Mallum when the Ancients left due to the threat of the Ori. In time, Amelius would go on to design another device, one that would have much greater significance, not only to the Ancients, but for the universe at large: the Stargate.
Later, the Ark of Truth would be used to defeat the Ori once and for all by Dr. Daniel Jackson, saving the Ancients and the Milky Way galaxy from falling under the Ori's control. (SG1: "The Ark of Truth")