The Great Stone was kept by monks in a tabernacle and it was believed to have magical properties. Uncharacteristically, its radiation had healing properties for those in desperate conditions. It once cured Machiste when he was dying from "The Black Death". However, The Great Stone could have the same detrimental effects as those of regular Kryptonite. For that reason, Deimos sought to acquire it to gain military advantage in his quest for world domination. According to Supergirl, just a moment of exposure would have been enough to kill her had she been at full power. Even having been drained by Skartaris' small red sun, it caused her great pain to be in its presence.
Warlord gathered an army to fight Deimos and keep the Stone safe, and with the help of a Justice League team, he succeeded. In order to keep the Great Stone away from evil hands, John Stewart permanently sealed the portal to Skartaris.