In the depths of outer space, a massive asteroid-like entity drifts idly above the planet Krull until it unexpectedly alters its path and descends upon its surface. As it lands, a terrifying swarm of formidable creatures known as Slayers emerges from the rocky fissures, unleashing chaos upon villages and cities. At the heart of this turmoil lies the sinister ruler of the Slayers, a gargantuan alien known as the Beast, who resides in the ominous Black Fortress. Ynyr, a mysterious elder, ominously shares a prophecy, foretelling that one day, a courageous young prince will rise to vanquish the Beast. Until such a time arrives, Krull remains trapped in a relentless cycle of fear and destruction.
The young Prince Colwyn, heralded as the potential savior of Krull, is set to marry his beloved, Lyssa. The prophecy further hints at the significance of Lyssa's ancient name, suggesting that together they will reign over the galaxy if they can conquer the Beast. As the wedding festivities commence at the castle of Lyssa's father, fate intervenes; just as the couple is about to ignite a magical flame as part of their ritual, the Slayers launch a brutal assault on the castle. Colwyn and his allies mount a brave defense, but they are significantly outmatched. In the ensuing chaos, Lyssa is captured, and both fathers are slain, leaving a wounded Colwyn presumed dead.
However, destiny has other plans, as Ynyr discovers the injured prince and takes him to safety to heal his wounds. Ynyr reveals the fierce responsibility resting on Colwyn's shoulders as the prophesized savior; he must rescue Lyssa from the clutches of the Beast, who has returned her to the formidable Black Fortress. Initially, Colwyn is skeptical, aware that the fortress is notorious for shifting locations with each sunrise on Krull, making it elusive and difficult to locate. Yet, unwavering, Ynyr believes that an ancient wizard known as the Emerald Seer possesses the ability to foresee its next appearance.
To fulfill the prophecy, Colwyn learns he will need a powerful weapon: the Glaive, a legendary five-bladed star weapon. The pair ventures to the perilous Granite Mountains, where Colwyn embarks on an arduous journey into a volcanic crevice to retrieve the Glaive from a stream of molten lava. After successfully extracting the weapon, he returns to Ynyr, who warns him to use it wisely.
While they make their way to the Emerald Seer, their journey is interrupted by the arrival of Ergo the Magnificent, an overambitious would-be sorcerer seeking to join forces with Colwyn and Ynyr after witnessing the threat posed by a menacing cyclops. As their quest continues, they encounter a band of released prisoners who, despite their pasts, show a sense of honor. Colwyn, recognizing their potential, frees them from their shackles and appoints their leader, Torquil, as his Lord Marshal.
Their journey leads them to the Emerald Seer’s abode, where they face a harrowing setback when the Beast crushes the Seer's emerald gem, thwarting his vision for their quest. In the gloomy depths of the Black Fortress, Lyssa endures the Beast's cruel taunts as he attempts to force her into marriage, threatening to murder more inhabitants of Krull if she refuses.
Amidst these trials, Ynyr seeks the advice of the Widow of the Web, a powerful entity who holds the key to predicting the fortress's movements. He courageously enters her lair, where memories of their tragic love resurface. The Widow reveals that the fortress will appear on the Iron Desert the following day but cautions Ynyr that his life is tied to the sand running through an hourglass gifted by her.
After a perilous confrontation in the Widow's lair, Ynyr returns to his companions, only to succumb to his injuries. With newfound determination, Colwyn and Torquil deduce that the fire mares, a mythical breed of horse, can carry them swiftly to the fortress. Just as they reach the Iron Desert, they are met with fierce opposition from the Slayers as they urgently ascend the towering fortress.
Inside, the group is separated, and while Colwyn battles the deadly traps protecting the chambers, he ultimately finds access to the center, where Lyssa is held captive. Wielding the Glaive, Colwyn prepares to confront the Beast, but he soon realizes the weapon will not return to him unless he draws upon the power of their wedding flame. Reminded by Lyssa, he unleashes its energy upon the Beast. In a climactic battle, Colwyn vanquishes the creature, breaking the traps that ensnared his friends.
As the remnants of the fortress disintegrate into the cosmos, Colwyn and Lyssa exchange vows, sealing their destinies as they rise together to rule the galaxy with benevolence, fulfilling the long-awaited prophecy.
