In outer space a large asteroid-like object floats above the planet Krull. It suddenly changes course and lands on the planet. From fissures in the rock pour out a vast army of strange and fearsome creatures called Slayers. They fan out and begin to terrorize villages and cities. The ruler of the Slayers and inhabitant of the asteroid, called the Black Fortress, is a hulking alien being called the Beast. A voice, belonging to an old man name Ynyr speaks of a prophecy that one day a young prince will possess the power to defeat the Beast. Until the prophecy can be fulfilled, Krull will endure years of terror from the Beast and his armies.
The young prince believed to be Krull's savior is Colwyn. He is betrothed to a young woman, Lyssa. The prophecy also speaks of the prince's bride being born with an ancient name and that she and her husband shall rule the universe if the Beast is defeated.
Colwyn rides to his bride's father's castle for the wedding. He and his father & their party are delayed because the roads they travel are dangerous and there are regular patrols of Slayers. When they arrive, the marriage ceremony is put into motion. Just as the bride and groom are about to complete a tradition of exchanging a flame the Slayers attack the castle and interrupt the ceremony. Colwyn and his father and his father-in-law-to-be mount a valiant defense but they are vastly outnumbered. Lyssa is captured, Colwyn's father and the bride's father are slain and Colwyn himself is wounded and left for dead.
Some time later Colwyn is found by Ynyr, who takes him to a hidden location to tend to his wounds. He tells Colwyn of the prophecy and that he is the prince that will save the planet. However, Colwyn must save Lyssa from the clutches of The Beast, who has taken her back to the Black Fortress. Colwyn scoffs at the plan since it is widely known that with each sunrise on the planet the Black Fortress changes location and never appears in the same place twice.
Ynyr seems undaunted by the ubiquitous nature of the Black Fortress -- he believes that and on wizard named the Emerald Seer can predict the next location. Ynyr also tells Colwyn that he'll need an ancient weapon to complete the mission: the Glaive, a five-bladed star-shaped device. To retrieve it the two travel to the Granite Mountains. Colwyn makes a difficult and dangerous climb into a volcanic crevice. He finds the Glaive lying in a stream of lava. He pulls it out and the sooty coating falls off, revealing an ornate weapon. When he returns to Ynyr, the old man tells him not to make use of it until he knows he needs it.
As they make their way to the Emerald Seer's home, they suddenly see a meteor-like object fly by. It turns out to be a man who introduces himself as Ergo the Magnificent, a would-be sorcerer. After a seemingly confrontational conversation, Ergo decides to join Colwyn and Ynyr when he sees a large and imposing cyclops.
Some time later the party encounters a small group of bandits who all still wear shackles indicating that they are actually escaped prisoners. Colwyn recognizes that they are criminals but not without honor and that they can help him in his quest. He frees them from their shackles and appoints their leader, Torquil, his new Lord Marshal. A short time later, Ergo is nearly killed by a Slayer; he is saved by the same cyclops he'd seen when he'd joined with Colwyn and Ynyr. The cyclops tells them his name is Rell. His race has a long history with the Beast -- they'd previously been a two-eyed race of giants who'd been offered the ability to see the future. However, the Beast had tricked them into giving up one of their eyes for the ability to see only their own deaths.
The band arrives at the Emerald Seer's home in a thick forest. The old man has a small apprentice whom is befriended by Ergo. The Seer uses a large emerald to predict the next location of the Black Fortress but is violently thwarted by the Beast, who crushes the gem from his location. The Seer says that he can try again from his own fortress deep in a nearby swamp where the Beast's power cannot reach.
In the Black Fortress, Lyssa wanders through many bizarre chambers. She is taunted by the grumbling voice of the Beast. When one of the hallways closes in on her, she enters the fortress' central chamber where The Beast himself lives. He tells her that she must marry him so he can secure his power. She refuses and he threatens to kill more of Krull's inhabitants until she agrees.
While traveling through the swamp Colwyn's band is attacked by Slayers, a distraction from the Beast, who sends a creature called a Changeling to kill the Seer and take his place. The Seer's body is found by Rell, who rushes to find the party. He stops the Changeling from killing Colwyn. With no seeming way to find the Black Fortress, the party settles in for the night in a nearby forest. One of the bandits' wives provides them with food. Ynyr tells the group that he will seek the counsel of an old woman, the Widow of the Web, who can also see into the future. When Torquil scoffs at the plan, citing the fact that no one has entered the Widow's lair and lived, Ynyr tells them that he knows the Widow's real name and he may be able to speak to her and live.
Ynyr enters the Widow's cave which contains a giant spider web. He is immediately pursued by the Widow's guardian, a large white spider. He calls out the Widow's name, which is the same as Colwyn's bride, Lyssa, and the Widow turns over an hourglass, stopping the spider until Ynyr can enter her chamber at the center of the web. Ynyr and the Widow have a history of unrequited love and the Widow reveals that she is trapped forever in the web because she killed the newborn son that resulted from their romance. She agrees to help Ynyr and tells him that the Black Fortress will appear the next day on the Iron Desert and will remain there until sunset. For Ynyr to leave the web alive, she gives him the sand from the hourglass. Ynyr cannot stop the sand from leaking out of his hands and the widow gravely warns him that his own life will run out when all the sand has run out. Ynyr is able to leave the cave and the spider attacks the Widow's home, destroying it.
Ynyr returns to the camp and bellows out the next location of the Black Fortress to the band, dying shortly after. Colwyn and Torquil puzzle over how to reach the fortress in time. Rell tells them that a species of horse, fire mares, can cross the distance. The band finds a herd of the animals and sets out, leaving Rell behind at his request: his own time of death is near and he wants to be left alone.
The band rides out to the Iron Desert just in time to catch the fortress. They fight Slayers while scaling the fortress' rocky exterior. Just when it seems they will not make it inside, Rell appears and holds a closing door open for them. After they are all inside, the door finally shuts, crushing Rell.
Inside the fortress, the band is separated by attacking Slayers. They encounter various traps in the many chambers of the fortress. Colwyn is able to find the central chamber where Lyssa is held captive. He uses the Glaive to cut a doorway, the process being very slow. Most of his band are able to avoid being killed by traps, however some of them die. When he finally gains entry, Colwyn again uses the Glaive to fight the Beast and finally lodges the weapon in it's chest. When he gestures to retrieve it, the Glaive will not return to his hand. The Beast attacks again, seemingly invincible until Lyssa reminds Colwyn that the power of the flame used during their marriage ceremony can be used as a weapon. Colwyn fires it at the Beast repeatedly until the monster is destroyed. The traps that ensnared his comrades deactivate and they join up to escape. Colwyn uses the flame to blast a hole in the wall of the fortress, allowing them to exit. The fortress sits on a lush, open field, and begins to crumble, the pieces of it flying into outer space.
Colwyn and Lyssa are officially married and we hear Ynyr's voice saying they shall rule the universe with benevolence.