The oldest Demon Lords of the Red Flame Kingdom were two, one named Mather, the other named Merry.
Mather was a unicorn demon, very serious, spoke little, but once he did, Demon Marshal Lames would seriously consider his opinions, and other demons would respect his thoughts.
He ruled over the Red Flame Kingdom's most important vassal state, the Gemstone Kingdom. That place produced spellstones and various associated gemstones; before him, the rulers of that place had been changed four times in a row, and none of them met a good end. This was because the spellstone business was so profitable that it involved multiple forces, especially the demon clans in the civilian population with hidden strength, including top-tier demon clans of the Demon Marshal rank.
When Lames appointed Mather as the new Duke of the Gemstone Kingdom, he did not have much goodwill. Lames had waged war with Mather, where Mather was the one who surrendered after defeat.