Hey there all, I'm setting up a short campaign based in a ruined undead filled city that has fallen underground. But trying to find a set of random encounter table for just undead creatures has been a hassle and even more so when it's for a group of starting characters. 

Any ideas from folks?

It's hard to make a random encounter table that works for anyone's specific game. The DM will end up customizing it enough that they might as well just roll their own.

Anyway, do you actually need one? What does it gain you over having a set of pregenerated encounters that you run in order? That way, you get more control over the variety and difficulty, when a random table could easily either throw repeated easy fights and bore your players, or several tough ones in a row and slaughter them.

If you really want one, I suggest separating the encounter types from the specific monsters. So, you might have:

• many weak opponents
• tough leader with weak minions
• a few medium opponents
• tough leader with a couple of medium minions
• two tough leaders
• etc.

And then separate tables for weak, medium, tough, etc. This way you can mix and match more easily.

The other thing I suggest, since low-level undead can be very generic, is that any monster can be undead if you say it is, give it the standard undead resistances and vulnerabilities, and describe it as such. The CR calculations might not be "right", but they're approximate at best to begin with.

Huh I never thought of that, that helps a lot thank you!

If nothing else, you can look at the random tables in Xanathar's and swap out things that don't make sense for things that do make sense

If you go into the Encounter Builder, you can use the filters to generate random encounter tables quite easily.  

Random encounter tables are fine for published modules; they give the impression that the world is not entirely under the DM's control, and that the characters aren't being railroaded. But for a homebrew game, they are a lot of work, and unrewarding.

If you make the table quickly (e.g. "2d6 Skeletons" or "3 Wights") then the encounter tends to feel weird. Why are they there? The DM has to construct a bunch of on-the-fly backstory for why these random monsters have just shown up, how they show up, and where they go after if the PCs avoid them. Alternatively, spend a good amount of time designing it ("2d6 skeletons in the armour of the old guard. They have XYZ on them. The skeletons only attack characters wearing pink headbands...") and 90% of the stuff you designed will never get seen.

They're a good tool for module builders, for whom thousands or millions of players will play in the game. For a homebrew game, just design the encounters you want to take place.

Hint, if you want to make undead encounters more fun, I suggest also thinking up/rolling up some random histories. They could be NPC's the players have previously met, they could be joke characters like a Butcher Skeleton with a butcher's knife, firing chicken bones at you, wearing a chef's hat rather than the standard short sword and bow (flavor only, no change in damage, etc.)

Fighting a zombie is often boring.  Fighting a child zombie carrying a teddy bear makes for a far more memorable encounter.   Warning, think about who you are playing with before you do this.

Your Flameskull could be singing Elton John songs as it attacks you.

That kind of thing really boosts your game's fun / cool factor without all that much more thought.  Just write down a list of 'histories' and cross them off as you use them up.

Thank you, that is another idea, considering the campaign is taking place in a underground city that is filled with it's former residents that works perfect.

My personal advice, as a dungeon master who has done a lot of wilderness/travel adventures, is to ditch the random encounter table entirely. In addition to being a hassle to prepare, random encounter tables, as was mentioned above, feel kind of bland. Encountering 1d6 + 2 skeletons out of the blue with no apparent purpose or motive and nothing to make the combat more interesting is the sort of encounter we generally try to avoid in D&D.

Instead, prepare a few planned encounters. Give each encounter motive, with a few monsters that have schemes or personalities. At the same time, make it flexible so the encounter can occur almost anywhere in your undead city, or at least in a specific part of it, when you decide it is appropriate. For even more exciting travel, create the encounters so that some of them are tied to the main storyline and/or each other, rather than being set apart from everything entirely. Finally, try giving the monsters a bit of treasure sometimes, because if there's one thing that is certain to motivate nearly any player you throw it at, it's the chance to get treasure.