I don't think I've ever seen a one-shot is purely written format before. If you don't know what a one-shot is, it's essentially a type of story (commonly in manga format) which is made as a preview to draw in support for a proper serialization -- in other words, it's a glimpse of a story in a small time-frame. It can also be a story which could be expanded, but isn't, and is rather crammed into a smaller format. This story reminds me of the latter type.
I think your story gives the minimum amount of descriptions as to what's happening in chronological order, which is commonplace for one-shots. I also think that, if you had taken this story and split each paragraph into its own part (excluding a couple, which would have been combined to make a "part", such as the first and second paragraphs), it would have been pretty darn good. This is also common for one-shots.
I'm not sure if you meant for your story to come out in such a way. If you did, then great. It's not exactly the best one-shot I've read, but if it was on a list of works to be fully realized (and thus expanded, in this story's case), it would likely have my vote for full serialization.
If you didn't mean for it to be a one-shot type, then it lacks detail. I don't know what any characters other than Arden (whose name I assume you changed within the middle of writing, because he's originally called "Ardal" on the 4th and 5th paragraphs, only to change halfway through the 5th) look like -- and even his appearance isn't very detailed, consisting only of the description of his armor: "thick plate armor".
Enemy encounters are completely non-existent other than Vanaduke, whose fight is omitted excluding the finishing blow and a single one of his attacks. The results are given, but the fights themselves aren't present.
Character development seems to occur closer to then end than anything else, but because the story didn't give much details on the personalities of the characters to begin with (other than short lines like "...Both of which were very cruel" or "Ardal wasn't much better off, one of his arms had been broken. Nevertheless, he grinned with confidence.", which were appreciated, but didn't exactly give a solid picture of the characters personalities), I can't tell how much of a change the characters went through beyond the labels "dead" and "still alive". The character that seemed to change the most was the Guildmaster's apprentice (who doesn't have a name), who went from cruel in the beginning to what appears to be more docile in the end -- but because I lack an adequate amount of details, I'm not sure whether she became docile. She could have just been sad over her master's loss, and would return to her cruel nature soon after.
There were also oddities in wording, like "They might kill me more". How does one get more killed than another? But, considering the length of the story, I'd say that these errors didn't amount to enough to look too deeply into.
The end reveals that this was all a story from the main-character, finally named Moren, which sort of explains the lack of detail as he could've been telling the story to some curious nub-knight running around Haven. Still, I think an ending like that could be considered common in One-Shots, and (once again) if you weren't going for a One-Shot feel, then I think the ending is kind of cliché.
That's not to say it's bad. Considering the length of the work and the amount of detail prevalent through the story up until that point, this ending is fitting. But still. There had to have been more things that occurred between the event and him becoming Guildmaster, and mentioning those details would have made the ending a bit better.
Even something like:
"We went back to haven, when my guild-mates noticed that their Guildmaster wasn't present (not even mentioning Arden), they fired off a barrage of questions, eventually leading to the question of "So who'll be the leader now?".
{Insert-Name-of-Apprentice-Here} was elected, but declined for {Insert-Reason-which-can-contribute-to-End-Development-Here}, and elected me instead. And that is how I became the second Guildmaster Moren, of Blank Faces, under direct order of Alex."
Side-note:
The first paragraph is in third-person, while the rest are in first. Considering the amount of writing, even if this was for creative purposes as a sort of "camera pans in on the main character" scene, first-person may have suited it better -- for consistency's sake, at least.
Final Notes/tl;dr:
As a One-Shot, this is a pretty good story. As a non-One-Shot, it lacks detail, even if it was meant to be a story told about the past to some stranger with a good dose of curiosity. Details and personalities need to be added to the characters in order to allow readers to better empathize with the characters, or at least visualize the scenes throughout the story. Without them, it's hard to get a good understanding of the feelings expressed during the situations presented.
This is a solid "skeleton of a story", and fits the "tale to another knight" or One-Shot feel pretty well. But it'd need to be fleshed out a bit more to make me, personally, feel anything for these characters beyond "Oh, he died. That sucks." or "Hey, he lived. Cool."