This post was inspired by a writing prompt in the Worldbuilding Community - Worldbuilding Prompt #1017 - Beserker
It's set in my homebrew Dungeons & Dragons world, and I've added a note at the end to show you the song that also inspired the post.
Enjoy !
Image created by AI in NightCafe Studio
Gravel scratched and scraped under the longship's keel as the crew pulled her up onto the riverbank. The waters of the Red Driva flowed past, the name a tribute to the blood red tint imparted by the scoria rocks of the Isvar Mountains, washed from their age-long sleep and swept downriver.
Their ship safely stowed, the crew followed their blond-maned handsome leader as he swaggered inland. Their destination was not far, and they reached it just before nightfall.
It was a tall, strongly build longhouse, eaves ornately carved with dragons, bears and wolves. The blond warrior called out, his voice ringing out loud and strong in the gathering twilight, "It is I, Gunther Steelblade, come to seek audience with Jarl Eirik Bloodeye !"
The black oak door with it's iron bindings creaked open to reveal an ancient bondsman.
"You've come to see Jarl Eirik, have you ? Another young fool seeking his blessing. Gundred Whassisname, or whatever you call yourself. Well, come on in, it's dark out and looks like rain."
The old retainer tottered into the longhouse, it's interior as dark as the growing dusk outside. A single fire burned, and a single torch. No warriors feasted, drank and caroused. No wenches squealed or servants hauled platters of roast meat.
Just a single figure on a wooden throne, dozing on the dais. White hair and white beard could not disguise a weathered and scarred face, nor age hide the talon grip of hands upon throne arms.
He opened his eyes with what seemed like the greatest effort and looked at young Gunther standing in supplication before him. You might have thought those eyes would be rheumy and clouded by age, but they weren't. They were translucent ice blue, clear and sharp as an eagle.
"So, you come to seek my blessing. I heard Falkar call you a young fool. Why do you seek me out ?"
Gunther hesitated, his arrogance and youthful bluster frightened away by the clarity of the terrifying old man looking down on him.
"My Jarl, you are the fiercest of all King Siggjan's warrior chieftains. Your reputation and courage are known the world over, your skill in battle unmatched. I.... I seek both your blessing and your knowledge."
"Knowledge ?" a snow white eyebrow rose, in mockery or curiosity.
"Forgive me, my Jarl, I heard you are the most ferocious berserker ever to have lived. I am leading my men to a perilous place, and would know your secret."
"Hah !" the jarl's laugh was a harsh bark. "You want my secret do you, boy ? Well there is no secret to it at all. Do you have faith in Odin ?"
"Well, yes," the young warrior replied.
"Wellyes," Eirik repeated in a sing-song mimicry. Then roared, "What do you know of faith, stripling ? Have you stood on a blood-soaked field when all was lost and cast yourself into Odin's hands ? Have you clung with iron grip to a longship's prow steering into the teeth of a storm and known that the storm god himself is at your side ? THAT is faith. Not 'wellyes'".
All Gunther could do was look down. He couldn't meet the eyes of this fearsome, terrifying old man. Yes, the Jarl was old, but he was old like blackened steel, old like the standing stone at the centre of the moor. Age had stripped him of none of his ferocity, for all that he spent his days sitting in his hall dreaming of battles past.
Then the Jarl softened slightly. "So tell me where your voyage leads you, young man. Is it a far place ?"
Gunther nodded. "We go far, far to the south, all the way down the Driva, to the Sarassid Lands. To the city of Dabirri, abandoned and overrun by the living dead, Draugr-like creatures. It is said the inhabitants died or fled and left vast treasures for those brave enough to seek them."
Jark Eirik nodded, a smile at last on his features. "A worthy quest. So I'll tell you. There is no great secret to berserkergang. It needs but two things. Faith and blood. That is why I am Eirik Bloodeye. Pray to Odin, and cast blood in your eyes, and the red rage will descend. I wish you luck in Dabirri, young Gunther. Bring me back a Draugr's skull to drink from."

When I saw this prompt, it reminded me of one of my all-time favourite songs, "Berserker" by Leslie Fish. She did a couple of versions of it, but this acoustic version is the one I prefer, even though it's maybe less polished than the version on the "Avalon is Risen" album. If you think about the lyrics, it'll make the hair on the back of your neck stand up....
https://youtu.be/0UMiR9UvJBs?si=91dDQSkaV9Q_2HRH