Constructive criticism you say? I'll do my best, but this might fall more into the realm of feedback.
Technical Aspects
Regarding how you write numbers, it seems to be a stylistic decision, but generally, almost always if the number is one digit you want to write out the word (i.e. five instead of 5). Figured I'd mention this, even if I didn't look into it too much.
I won't repeat the dialogue punctuation thing Cricket covered, but I did mention it just now so that there is a reference to it in this post as well.
There might be something in regards to structuring sentences in a certain way, something like what makes a complete sentence, but I'm pretty ignorant in this department (for now), so I won't try to guess about it and assume it is probably just your style/writing voice that I'm noticing (since it is different to mine). I should say that the writing does look good to me, so I'll just throw in a friendly reminder to not forget about proofreading once your larger work is completed.
Plot, Characters, and most importantly, Stakes
Eh, not much to say regarding the plot. Obviously, a capital being overtaken by a four month old undead horde is bad, but as there is a lot of ways for the story to go I can't really comment on how good the meat of the narrative will be. Heck, it might end up being more focused on the characters and their growth, I just can't know at the moment.
As for the characters, you do introduce all five and set up a common goal for the party, and seeing as they appear to be going to deal with the undead, there are stakes here, even if they haven't really been developed yet.
Speaking of stakes, I'd advise to at least somewhat establish what will happen if the party fails. Not necessarily here in the opening, but do keep it in mind for latter scenes. This is because, so far, as I ought to make the least assumptions for a story that is only beginning, I'm assuming that if the party fails, it is bad for the city and themselves. Currently however, this is lacking.
This is because a capital city I know of only by name is not something I'm very invested in, especially because it has already been overrun, so a party fail won't really impact it, it'll just remain overrun.
So what about the characters? I cannot really be invested in them from just this introduction alone, as there isn't enough to make me really care about any of them. Don't get me wrong, I'm not rooting for them to fail or anything, in fact I'm probably leaning towards liking them since there seems to be some sort of party dynamic going on, but (and maybe it is just me) I need more to get invested in characters.
Now, this can somewhat obviously be remedied just by writing more, but I am still mentioning it specifically to draw attention to it, because it is all to easy and possible to neglect putting thought into why and how readers will be engaged, in this case I'm focusing on stakes, because in this case, if I care about the party, I obviously don't want them to fail, and so I am more engaged.
Anyway, I will say that depending on what type of story you're going for, i.e. more character driven vs more event/world driven, you might want to find some way to make me care about this city which has been overrun. Probably just finding a way to let readers learn more about the city would work, but be careful to not end up with a dull exposition dump. For something as large as a capital city, I imagine there is more than one way to make readers care about it, so just consider how important the city itself is for your narrative, and ensure readers understand this so they remain appropriately engaged.
I'll finish this section by mentioning two (err, three) things:
- It is possible to give readers both a reason to care for the characters and the city, but I'm not sure this is necessary, more so something that depends on the story your trying to tell.
- Further, all this really does depend on what you're going for, so while I do try to be specific in spots, some of my advice might not be helpful for your specific story. It depends on so much, so at least keep the general idea of keeping readers engaged as important, since there are many fantasy stories out there, and this can lead to them feeling uninspired or dull, which is something I think is worth actively trying to avoid (for obvious reasons, namely, not wanting readers to be bored reading your work).
- I mostly focused on stakes for engagement because stakes, in general, are something I struggle with, so really, I'm writing this for myself as much as I am for you.
Conclusion + TL;DR
Keep writing, it looks good. Many ways for the story to go, so I'm curious to see how this turns out.
P.S. "Short piece that I've written" + "the story is a small section of what will eventually be a larger text" makes me assume this is not a storygame, but I also want to mention that you'll probably want something to happen regarding some form of a conflict/contrast if you want this to end up as a complete story. Right now it is just an opening, you do establish stuff, but that is all. I will say that there are obviously numerous ways to write/structure a story, but what you have so far feels lacking to me (in regards to being a whole work).
I'm only mentioning this in a post script because of the second excerpt, which I read with more emphasis on the "be a larger text" as opposed to "the story". As such, I could have misunderstood and you are talking more about this being almost a part one of a larger collection, or maybe short stories that are linked together, in which case I would have more feedback to give regarding my thoughts on this as a standalone story (mainly in regards to structure), which is why I have this post script at all (you know, other than the fact that almost all my longer posts have one for some reason).
P.P.S. Oh, almost forgot, if there is something more specific you want feedback on, feel free to ask/clarify. Also correct me if I got anything wrong. Hope you got something useful from all this.
EDIT P.P.P.S. Messing around with formatting so that these longer posts look nicer... I have some ways to go.