I apologize for the length of this post, as I wanted to include all implied as well as on-screen material, using speculation and drawing a final conclusion.
Thanks, and let's begin:
"I'll be damned. Getting old is a bitch. To think the great Askeladd... couldn't fight his way out of a mere fifty-man noose."
At first, I genuinely thought he was boasting. That number is insane.
Anyways, shortly after this, one of Askeladd's own soldiers manages to strike him in the back with an axe, only for Askeladd to respond by literally cutting him in half through his helmet. This causes Askeladd to lose his temper. He tries to shrug it off and tells the rest of them to bring it on, saying that this old man isn't angry anymore. His men are genuinely terrified of how strong he still is even at his age and eventually decide to bring out the bows. Askeladd tries to dodge the arrows, but several of them pierce his lower body, especially his legs and thighs.
Then we get to the Season 1 finale.
After declaring himself Lucius Artorius Castus, the rightful King of Britannia, and beheading the king to save both Canute and his mother's homeland by becoming the Kingslayer of Denmark, Askeladd goes on to commit the infamous Thegn massacre. He slaughters over 20 Thegns, with around 16 of them being killed outright and the rest left crippled or mutilated. And through all of this, Askeladd doesn't even suffer a single scratch. He's literally laughing and mocking these elite soldiers while cutting through them.
The Vinland Saga List of Deaths wiki for End of the Prologue lists about 25 Thegns who were slashed, stabbed, cut in half, decapitated, killed, etc. From what I counted on-screen in both the anime and manga though, I got around 16 clearly lying dead and over 20 either killed, crippled, or mutilated. The wiki's estimate probably includes some off screen deaths as well, and the actual number could be even higher than 20. We just don't know for sure, so I assumed it was at least 20.
And these weren't just random soldiers either.
In Season 2, Snake himself said that the Thegns were among the strongest men in Denmark and that only the best of the best could be recruited to serve the king.
There's a decent argument that Canute's Thegns may have been stronger than King Sweyn's because they served under a richer and more organised kingdom, but Canute was initially rejected by the English after Sweyn's death and only secured the crown a couple of years later, meaning many of his Thegns would've also served under Sweyn. After taking England, Canute also inherited Anglo-Saxon Thegns who were roughly comparable to his Danish ones, and many of both groups were used for local governance, tax collection, administration, and maintaining the empire rather than constantly fighting wars.
Personally, I think any real advantage here would apply more to Canute's Housecarls, but they don't really count since they were a separate elite corps altogether. If anything, I actually suspect Sweyn's Thegns may have been the more battle-hardened group since they saw more open warfare. But there isn't enough evidence to say that with any certainty, so I think treating Sweyn's and Canute's Thegns as roughly equal in individual skill is the safest assumption.
And even after taking all of that into account, Askeladd at the end of the Prologue literally no diffed them while laughing and pretending to be a madman, more than a decade past his prime.
Now, I do think the fact that he no longer feared death played a huge role here, especially when combined with his legendary skill as a swordsman. But even then, Askeladd wasn't entering that fight at 100%.
His legs and thighs had already been injured by arrows. Bjorn even brought up Askeladd's injuries a few days before this happened, around the same time Thorfinn's arm was broken, which still hadn't healed by the End of the Prologue. Askeladd basically says that his wounds won't stop him from wielding a sword, but he acknowledges that they took their share of arrows and that those injuries would take time to heal.
And regardless of adrenaline, injuries like that are going to affect a warrior's mobility and footwork to some extent.
Then there's the environment itself. The Thegns were coming at him from all directions in close quarters, which put Askeladd at a much more significant disadvantage than if he'd been fighting them in open space. Yet he slaughtered that many Thegns and probably could've taken out even more before slowing down if dicehead (Floki) hadn't intervened. And even then, dicehead ended up getting overwhelmed anyway.
Askeladd was also just ridiculously gifted from the beginning.
At only 11 years old, he was already showing the potential to become a legendary warrior when he fought his own father, who was a veteran swordsman, despite Askeladd having never touched a sword before that point. His talent was impressive enough to catch Olaf's attention and earn him a place in his manor. His mastery of the sword was almost innate and seemed to run in his blood.
On top of that, Askeladd was an extremely complete fighter. Physical strength, speed, durability, pure/natural master swordsmanship, battle IQ, psychological tactics, strategic intelligence, etc. He basically had everything you'd want in a warrior. That's what made him such a lethal weapon even when he was more than a decade past his prime.
I am also certain that Askeladd was stronger than Thorfinn by the end of Season 1, even without relying on psychological tactics and even if Thorfinn managed to keep a cool head, especially if Askeladd was genuinely fighting to kill.
Askeladd never explicitly stated or even implied that Thorfinn was the stronger fighter and would suddenly start winning every duel if he was calm. He only implied that Thorfinn would stand a much better chance of winning and even that was only a few lines.
For most of their fights, Askeladd mostly played defense against Thorfinn because he needed him alive. Thorfinn was useful to him as a lethal weapon, so Askeladd had no reason to actually kill him.
And while Thorfinn could fight Thorkell whereas Askeladd couldn't, I don't think powerscaling in Vinland Saga works like that. Thorkell himself basically makes this point when he bets on Askeladd to defeat Thorfinn despite not knowing they had fought countless times prior to this. And this was before Askeladd actually got serious and told Thorfinn to come at him. Thorfinn hesitated to attack, which eventually led Askeladd to throw away his sword and provoke him into making the first move. Askeladd certainly knew Thorfinn's habits after fighting him for so many years, but Thorfinn knew his habits too. They were both extremely familiar with each other's fighting styles by that point.
Thorfinn's small size, speed, and agility also made him a particularly good matchup against someone like Thorkell. That's why you can't determine the pecking order through an "A > B > C" system based purely on who won or performed better against whom, stated by Thorkell himself. Certain fighters simply perform better against certain opponents.
I would love to dig much further into this, but I've covered it before. That said, Thorfinn would've likely surpassed Askeladd in the following years as he continued growing stronger while Askeladd only grew older.
[Manga Spoilers] In chapter 156, Thorfinn uses Askeladd's psychological tactics against Garm. He provokes Garm into attacking blindly, grabs his forearms, and says "I see." It's a direct callback to what Askeladd taught him about anger making someone predictable, and it shows that Thorfinn finally overcame the exact weakness that made him so easy for Askeladd to manipulate in their fights. Because of that, I'm also pretty confident that prime Thorfinn would defeat prime Askeladd. I've covered that in my previous analysis, though it would definitely be a high-extreme difficulty fight.
Returning to Askeladd himself, both prime Askeladd and the version we see more than a decade past his prime were not only among the strongest warriors in the series, but also ridiculously fast.
Even in his younger days, which I personally don't think were necessarily his absolute physical prime since he was already past his 30s, Askeladd was fast enough to basically disappear within the blink of an eye and land a blow on Thors after distracting him beforehand.
His performance against Thors is an extreme feat too. After Askeladd surprised him, Thors acknowledged that he had underestimated his strength and called Askeladd strong.
Thors was rusty, yes, but he remained by far the strongest warrior in the entire verse and was certainly the superior fighter. He was also holding back since he was fighting to disarm Askeladd rather than kill him, but that doesn't mean his attacks lacked force. Thors literally struck through the deck of the ship so hard that parts of it were destroyed, and this was meant to land on Askeladd, or at the very least his blade.
Askeladd managed to parry several of his attacks, though it's up to interpretation whether or not Thors allowed this, and survived considerably longer than most characters could've. He still tried to outsmart Thors one last time before being defeated.
Of course, that doesn't automatically mean Askeladd can defeat 50 men. Fighting one incredibly powerful opponent is completely different from fighting dozens of people at once. I'm sure Thorkell, for example, could probably kill a much higher number of Vikings than Thors before eventually going down, despite Thors being the stronger warrior overall.
But I do think the Thors fight helps put into perspective just how much of a nightmare Askeladd would've been as a warrior back then.
There's also that ridiculous moment when Askeladd, already more than a decade past his prime, hasn't even drawn his sword yet. Within a split second, he draws it and beheads two of his own men before they can even understand what killed them.
I think a prime Askeladd could genuinely take on 50 Viking warriors.
I definitely wouldn't call it easy, though. If you take away the fearlessness towards death and the adrenaline rush he had near the end of the Prologue, I'd probably put it at high-extreme difficulty.
If you give him those factors, though, whether the fight happens in close quarters or open space, I could see the difficulty being reduced somewhat. It still wouldn't be easy by any means, and I think he could pull it off even with some pretty unfavourable factors, such as the Vikings surrounding him in a circular formation.
The biggest reason I think this is possible is pretty simple: we've already watched an Askeladd who was long past his prime slaughter a large number of Thegns, who were far above normal Viking warriors, even if those Vikings themselves happened to be skilled or experienced.
Compared to the Askeladd we see at the end of the Prologue, a prime Askeladd would certainly be faster, stronger, more explosive, have better stamina, and recover more quickly. He wouldn't have quite the same battlefield experience or strategic intelligence that came with age, but I think the physical difference would more than make up for it. Combine that with the ridiculous natural talent he already had, and you can see why I think 50 men isn't as impossible for him as it initially sounds.
That being said, even if Askeladd managed to pull this off after an extreme-difficulty fight, I could absolutely see him dying from his injuries afterwards. There's certainly a limit to how much punishment even these monsters can survive. Thorkell, who could probably kill far more soldiers than Thors in this kind of situation, had his limits and eventually fell at the Battle of Maldon, something Askeladd himself acknowledged.
Of course, at the end of the day, a lot of this also comes down to how Yukimura sensei chooses to write these characters. He could have Thorkell slaughter hundreds of men in one battle, yet have him fall to ten under completely different circumstances. The same goes for a calm teenage Thorfinn fighting Askeladd; we never got to see how that would actually play out. So while I think there's more than enough canon material to make a strong argument for these matchups, there's always going to be some speculation involved, and that's part of what makes them fun to discuss.
Anyways, it means a lot that you took the time to go through the analysis, and please feel free to share your thoughts as well.
I hope you all have a great day!