Alexander Skarsgård rages as a Viking berserker in The Northman trailer

"I will avenge you, father. I will save you, mother. I will kill you, Fjolnir."

These are the only tasks on Alexander Skarsgård's mind as the True Blood and Big Little Lies star transforms into a vengeance-seeking Viking in the first trailer for The Northman.

Robert Eggers, who directed smaller, more confined indie films The Witch and The Lighthouse, tells EW over a call from London, where he's editing the movie, that this epic saga is "by far the hardest thing I've ever done and by far the most ambitious."

It's a story told through the eyes of Amleth (Skarsgård), who witnesses the murder of his father, King Aurvandil (Ethan Hawke), at the hands of his uncle, Fjolnir (Claes Bang). With his mother, Queen Gudrun (Nicole Kidman), captured, Amleth flees his native land. But, like a beaten Arya Stark reading aloud her hit-list every night, he vows revenge on the one who took everything from him.

Aidan Monaghan/Focus Features Alexander Skarsgard as Amleth in 'The Northman'

"It's basically the story of Hamlet or The Lion King, which was originally an old Norse story where the Hamlet character was called Amleth. We've told it in our own way," Eggers says. "It is a Viking saga with magic and swords and mysticism and all the stuff that you'd want from that."

The Witch, released in 2015, made indie stars of both Eggers and Anya Taylor-Joy (who also appears in The Northman). But after years of failing to get larger films off the ground, the filmmaker made something "small and deliberately oblique": The Lighthouse with Robert Pattinson and Willem Dafoe. That was meant, he says, "to get my frustration out."

Eggers then found a kindred spirit in Skarsgård, who had been trying to make a Viking movie for some time to no avail. The man even looked like a Viking. "I mainly remember how tall he was," Eggers says, chuckling at the thought of that first meeting. "At the end when we got up to shake hands, it took him about 30 seconds to stand up completely. He was so tall."

Aidan Monaghan/Focus Features Ethan Hawke as King Aurvandil in 'The Northman'

The writer-director already had a Viking story in mind, the concept that would become The Northman. Eggers explains it in relation to a certain beloved Halloween family film: "Hocus Pocus had kind of nailed the last nail in the coffin of witches being so scary. With The Witch, I wanted to say, 'Actually, let me show you what witches are for real.'" The Northman is the same idea. "People think that they know Vikings. They think of the Kirk Douglas Viking movie [1958's The Vikings]. They think cartoon guys with horn helmets or certain TV shows where Vikings dress like science-fiction rock stars. This is something very different."

Aidan Monaghan/Focus Features Nicole Kidman as Queen Gudrun in 'The Northman'

Eggers called up a friend, the Icelandic novelist and poet Sjón, to see if he would write the screenplay with him. He had met Sjón previously through, wait for it... Björk. The internationally renowned singer introduced them at a dinner party, thinking they'd get along. "She was right," Eggers adds.

That's also how Björk landed a part in The Northman. Both Eggers and Sjón couldn't think of anyone else to fill the role of Seeress. ("Remember for whom you shed your last teardrop," the character utters in the trailer.) Eggers makes note of Björk's "magical charisma," adding, "She completely is her persona."

Aidan Monaghan/Focus Features Alexander Skarsgård as Amleth and Anya Taylor-Joy as Olga in 'The Northman'

Taylor-Joy reunites with her Witch director as a different kind of enchantress, Olga, a character Eggers prefers to keep somewhat a mystery.

"Your strength breaks men's bones. I have the cunning to break their minds," Olga tells Amleth.

Cryptically, Eggers mentions, "Anya is a good witch in art and in life."

Dafoe, another of Eggers' former actors, appears as Heimir The Fool. But of all the faces among the cast, it is Skarsgård who carries the film.

Focus Features 'The Northman' poster featuring Alexander Skarsgard as Amleth

"Alex totally transformed his body," Eggers says. "He's bigger than he is in Tarzan, and that's no small feat. He completely became this character. When he first had to do that kind berserker transformation scene" — the actor's raging, bloody sequence glimpsed in the trailer — "everything fell into place. To my taste and understanding, he is like a saga hero. There are moments where his rage is as powerful as anything I can think of in cinema. I'm not trying to toot my horn, but it's intense. I feel ridiculous saying something like that, but it is intense."

The Northman is scheduled to open next year on April 22.

Related content:

• Björk joins Nicole Kidman in Robert Eggers' Viking movie The Northman

• Alexander Skarsgård, Nicole Kidman eye reunion in The Witch director's next film

• Robert Pattinson and Willem Dafoe dance macabre in masterful drama The Lighthouse