Besides being king, Frontarius was also proficient in theoretical magic, his skill being in determining the ebb and flow of magical currents, knowing where the potential for power was greatest.
He travelled the sea for many years, seeking where the highest peak of magical power in the world might be.
After much travelling he found an ice field where the peaks seemed highest—we know where it is, it is called the steps of Frontarius—and from each peak he found a higher and still a higher after that, till he finally found his summit.
And at that point the nightmares gathered.