Hello everyone! I am really new to D&D and even newer to being a DM. I ran a short game for my sister and a friend one time but otherwise I have no experience. For my birthday I am planning to run a one-shot for my family and 3 of my friends. I have the idea for a story coming together, and I want to make an encounter involving the city's local thieves guild. I do not own the Monster Manual so I tried to look on here for an NPC that could be used for the thieves I want. The Spy was the closest I could find and that seems fairly decent, but I'm worried because it says that their AC is 12 and they have 27 HP, and I feel like that is weak. I put the originally planned encounter into KFC (6 level 3 PCs vs. 4 Spies) and it said that it was only a medium encounter. I would love for this encounter to be either the final fight or one of the later ones in this one-shot experience, so I want it to be memorable without a TPK. This is where I need your help. I feel as if I have a few possibilities. I could up the number of spies (5 apparently becomes a Hard encounter, while 7 makes it deadly), but I am worried that this will be too much for me to keep track of. I suppose I could simply up the AC and HP a small bit, but I have never tried that before so I do not know how far to go with this before it becomes unbalanced. The third option I considered would be just rolling up 4 rogues on my own time, but again I am not sure how this might balance out. (The concept of CR ratings is still hard for me to grasp). Would 4 level 2 rogues be a good match against 6 level 3 players? What would you do in this situation? I do not yet know what classes/races my players will be yet because we are still a month out and I told them to take some time to decide. I have added a poll to this but any comments or advice would be welcome. Please feel free to ask questions. Thank you most sincerely for all your help!!

~Goggles

I would totally craft the environment to make it so the spies have the advantage.  Have traps, throw in some commoner hostages that might sully up combat, have it in a burning building, or basically just add in environmental factors such as darkness, mist, fog, noise, or something that'll make use of the spies' sneak attack.  If distance will help, add in some obstacles.  Maybe don't even reveal a couple of the spies until combat is well engaged.  You could also give a couple of the spies some magic items that'll help them.  Browse through some of the wondrous items and find something fun for them to use! 

Biggest deal is to look at how much damage the PCs are going to be doing each round as well as what the PCs ACs are. This can be tricky with spells and such. Remember the previous encounter alters how difficult the current encounter is as the PCs have less resources at hand. Also the PCs attack bonus is important to notice. If the average PC deals 20 damage a round, then with 6 PCs dealing roughly 20 damage (totally making the 20 damage up, I didn't look to see what 6 lvl 3 PCs could do each round) and have a +5 to attack (+3 ability score, +2 prof.) So on a roll of 7+ the PCs hit. Roughly 2/3 of the PCs will hit, 1/3 will miss. Knowing the PCs actual classes and how they are going to use their classes would help, but simplified we will assume each PC makes an attack. SO 4 PCs hit, 2 miss. 

On average it looks like the Spies will roughly hit 50% of the time.

PCs vs 4 Spies. in PCs Favor.

PCS win initiative. Round one, two spies die without attacking (or good rolls/ abilities they all might die without attacking). The two spies left alive hit two of the six PCs dealing 8 points max. (5 avg). At lvl 3 everyone should be alive, someone might actually die if a sneak attack is able to get off. Round Two the other remaining Spies die without attacking.

PCs vs 4 spies, Spy favor.

Spies win initiative and get off sneak attack

Max Damage with Sneak attack = 20. Probably enough to kill a PC. At average Damage Probably not.

Likely only two or three will survive the first round, meaning they probably won't be able to get off a sneak attack in round two.

All this to say that the Spies are a good challenge for the PCs, especially if they haven't taken a long rest before the encounter. Bump up their HP beyond the average number and maybe give them some leather armor since they aren't actually spies. And with some good tactics the encounter can actually become quite difficult. This all depends one the PCs classes and spells and how long ago they rested.

Go for 2 Bandit captain (450 x 2 = 900 xp) and two spy (200 x 2 = 400 xp) (total=1200 x 2 = 2400 xp, the party threshold for a difficult encounter is 2700) . It is an almost difficult encounter. If you fell that is going easy, boost the hp.

Generally, when generating an encounter, I feel a better balance when there is a "boss" and few low CR NPCs.

Welcome to the hobby Goggles! And good luck as a DM!

There's a few things I would suggest:

#1 - Are there any new players? If you're doing this with family or people not used to the system, make it just level 1 PC's to make it simpler for them. Right away that's going to up the difficulty. If there's anyone that already has a level 3 character, let them use it and have them be the leaders for everyone else, but the monsters know of these more legendary characters and will try to single them out (making sure to explain "It's Rogthar the Barbarian! Let's get 'im and the boss will greatly reward us!").

#2 - Don't make it too big. One-shots almost always go over the time limit, especially with new players. Make it, like, 3 encounters long. One encounter where they get surprised, another where they investigate, and a third where they confront the big-bad.

#3 - Give the enemy monsters an edge. Specifically, try to make it so that the players fight at disadvantage, or the enemies fight at advantage, or both! However, the trick will be in crafting it so that the players can figure out a way to remove those penalties, or even turn the tide around. You don't necessarily have to come up with how they can remove it, just make it something that isn't just "these creatures fight at advantage". There needs to be a reason that can be discovered, and the players will come up with ingenious ways to overcome it.

Wow, thank you for this advice! I totally didn't even think about adding environmental factors into it. I really like the idea of hiding some of the spies too. Also I have no idea why I didn't consider giving them items. Sometimes I forget that bad guys can be smart too. The bandit captain thing is really brilliant and I can't believe I missed it. The breakdown on what will happen based on initiative is super handy as well.

One thing that I saw brought up was the PCs being level 3. Most of my players are new and if this was a full campaign I would definitely start them out at 1, but I was worried about them being squishy and not having fun because they couldn't do a whole lot. I had considered starting them at Level 5 but I realized that that would be far too advanced for the newer players. The thing is however is that I know that there's no way this will be able to turn into a full campaign. With everyone's work schedules and some of us going off to school, it just won't be possible (thus why I am doing this for my birthday, it's the only way I can get everyone together). I am worried about this possibly being the only experience some of these people will have with the game, at least for a while. Given all of these factors, do you still think it is advisable to start them off at Level 1? What about Level 2? I do have my regular DM playing in this game so he can certainly be Level 3, and my baby sister has played once or twice, so she could probably handle it too, especially since I'll have more time to work with her than just about anyone else. What do you guys think?

I am so unbelievably grateful for all of you guys' help. The responses were so informative and it makes me happy that you guys are so willing to help new DMs like me learn and work things out. You guys are the best!

Definitely start at level 1. For one thing, it's a complicated game. Speaking for my own family members; they won't quite "get it" the first time 'round. As such, level 1 means they have less to focus on. They may have just a few abilities, and if any of them want to be a wizard, they will have very few spells. Options are great, but you just want to tease that there's more options out there.

Think of it this way; when you picked up your favourite computer/console game, did you start the game with full access to everything? Of course not! You were given limited access because you had to learn as you went. Well, same here. At level 1 they'll have limited access. Yes, they will be squishy, but there's lots of level 1 classic monsters that are equally squishy (zombies, skeletons, goblins, KOBOLDs, humans), with most of them also having lots of ways to customize and flavour them without changing a single stat, or that can be made very interesting by giving them different gear.

Squishy can be solved rather easy. Just say everyone gains their hit (die max + con) x2 hit points. Third level doesn't solve the squishiness that much as if you get a d6 you'll only have about 12 hit points on average. This means you aren't likely to die in the first round, but the second round still becomes deadly.

Third level works fine if you provide pre-gen characters. Just make one or two of every class. I don't think there is any class where third level gets super complicated. And most spell casters are really only gaining roughly 1 or 2 more spells. Third does allow the hybrid classes to gain their spells and I believe will give each class their signature abilities. So I can see the benefit of running third level.

As far as RPGs go, D&D is relatively easy so I think there isn't going to be a huge difference in running at third level as first level. You don't want to be running at like 10th level, but starting at third shouldn't provide any more difficulty than starting at first. I don't think I would start at level 5 as that is the point the classes start getting more complicated. I really don't think level three is going to be that big of a difference. Could add a little bit of time to making characters.

Pre-gen characters would I think relieve most of the difficulty with starting later. The real benefit is that third level allows the PCs to make more mistakes than 1st level, especially if the encounters are tailored to be less challenging to allow them to get use to the game.

D&D is an unbalanced game that is impossible to balance.   period.  Everything from terrain, to saving throws , to a tired player can turn a "tough" fight into a "cakewalk" or an "easy" into a near TPK. And a  one-shot adventure where you don't have experience with the players is even more difficult.  

I many times don't set a hard number.  If i think 4 is the right challenge, i start with 3, then have 0-4 more show up depending on how the fight is going.   

I have a fight coming next week v. 4th lvl characters that has 8 to 42 Orcs.  Running fight through the woods, and it will seem to the PC's that they are unlucky to keep stumbling into other patrols, but it is not dice, it is what makes the encounter feel like it pushes them to the limit without crossing into TPK range.  

Treasure and xp is a quick multiplication of "each" x number actually defeated.  

I feel like every party I've ever played in would stop and attempt to rest to regain hit points or hide and wait for the orcs to move away.