Eyjafjallajökull and Mýrdalsjökull are enormous glaciers on Iceland’s South Coast; they’re thought to be sisters of one another. Mýrdalsjökull, however, covers one of Iceland’s largest and most active volcanoes, Katla.

Icelanders fear Katla for both its size and proximity to human settlements, but there’s also a third, more insidious reason: Katla is also the evil sorceress that plunged down into the caldera over 800 years ago.

Katla the sorceress was employed as a housekeeper at Þykkvabæjarklaustur, a monastery on the South Coast, in 1169. According to the legend, she was both ill-tempered and cruel.

Katla owned a pair of breeches that would allow anyone wearing them to run without feeling tired, and she used them when needed. She was feared by all—even the abbot who employed her—because of her supernatural powers and foul temperament.

A young shepherd boy named Barði often bore the brunt of Katla’s temper, particularly if a sheep strayed. One autumn night, Katla and the abbot attended a feast; Barði was ordered to bring the sheep in from the surrounding hillside before they returned.

However, Barði couldn’t find all the sheep. Fearing Katla’s rage, he snatched her breeches to speed up the process of collecting the rest of the animals. When he returned with the flock, Katla was waiting for him.

She grabbed the boy and drowned him in a tub of whey acid which, in those days, was used in food preservation. She left his body there throughout the autumn, and when asked about Barði, she would simply reply that he’d show up soon enough.

And soon enough he did: in midwinter, the tub disintegrated, and Barði’s remains were found. Katla was summoned to the authorities, but before justice could be served, she grabbed her breeches and ran toward the glacier where she plunged into the caldera and was never seen again.

However, shortly after she dove into the magma chamber, the volcano began to erupt. The glacial flood that followed headed straight for the Þykkvabæjarklaustur monastery. All of the locals knew this was Katla’s vengeance at work.

To this day, whenever the volcano threatens to erupt, Icelanders tremble at the thought that the evil sorceress will return.