A creature named Lavos. It convinced the Queen of Zeal that it could bring her and her people into immortality and she lost her soul to it. When the kingdom fell, and quite literally at that, most of the magic users were killed. Of course, with the subsequent tsunami, many of the survivors - both magical and not - were killed. After that, the magic users lost their connection to Lavos and their magic. It was thought to be lost after that, but there are a few who have sufficient Enlightened blood to use it still.