The Children of the Altar: Born of Sacrifice

On a distant planet far from its sun, the planet Wrüm remained warm only because fire mages constantly regulated its global temperature. Magic is inherited if at least one parent is a mage, but true mastery of an element becomes accessible only when a mage of that element is sacrificed on its altar, empowering all births in its range. When fire mages refused further sacrifices, the world fell into an eternal winter that wiped out the water mages and fire mages.