Others have mentioned other reasons, but here's another that I find particularly compelling.
In a game of D&D, it is generally accepted that the DM will build combat encounters to be difficult for the characters no matter what level they are at. This can lead to a pattern where your players feel as though, even with all the new powers they are getting, ...
The players may have gotten out of the boss fight I had intended
Excellent - I love it when players outsmart me, giving players the chance to feel clever and empowered is what being a DM is all about.
Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win.
Take a step back and ask: what is ...
Play the monster like the ambush predator it is
If a monster is slow and has no ranged attacks, then it is most likely an ambush predator that relies on the element of surprise to catch its prey. This seems to be the case for a chuul, which has many abilities well-suited to ambushing adventurers: it can sense magic (including the magic items that ...
AngryGM Says Something About This
Once your characters are obviously going to win, end the encounter/fight. That's tough, but I'm going to sum up what he said. (You should still read it, though) You need to figure out what the main question the encounter is trying to answer, and when the answer becomes obvious, end the encounter! Yes, I know AngryDM's ...
Talk to the DM
Give non-accusatory feedback
Express your point of view using neutral language and I statements. E.g. "When the encounter with 9 ogres ended in the death of all the characters, I felt helpless and it was not fun for me."
Ask for feedback.
Ask them if there were alternative endings they had in mind. It can take a while to get in sync with ...
There are many ways this could be explained, here are a few good story drivers that work:
Different areas have stronger and stronger enemies
Different areas in the world grow rougher and tougher as they stray from mainstream civilization. You can draw your party out into the wilderness for whatever quests they obtain.
A low level party would not be ...
at the same time I have a hard time understanding exactly what else they would do.
That is the core of your problem. To make them interesting, or boring but "alive" you must understand why they are doing whatever it is they are doing.
4 goblins ambushing caravan may have many different motivations. From the top of my head:
Hungry families they need to ...
A fair number of spells prevent teleportation; a wizard will struggle to gain access to some of them, but the skill Use Magic Device and a wand or staff will solve that.
The 4th-level Sor/Wiz spell dimensional anchor [abjur] (Player's Handbook 221) for 1 min./level prevents 1 creature from using any extradimensional movement if a ranged touch attack ...
Treat difficult social encounters like combat
Any encounter shouldn't be finished in one roll. The bandits guarding the bridge aren't going to roll over and let the party pass just because you asked nicely once, but you may be able to persuade them to not be initially hostile, deceive them into thinking you've got nothing worth stealing, then with some ...
Telegraph the problem in advance
It sounds like the real danger here is that the group might communicate sincerely with the spymaster, and this could lead to the spymaster killing them all. You can avoid this by letting the group discover evidence that the spymaster is treacherous -- perhaps people they talk to can complain that the spymaster is using ...
CR and Encounter Building are not an exact science
If you find that your group is too effective (or too ineffective) in dealing with enemies, you will have to improvise and adjust the difficulty.
Homogeneous groups share weaknesses
Shambling Mounds lack ranged attacks and are slow. A single character able to attack at range can defeat any amount of ...
Don't make the boss the target until after the minions die.
Some of the most memorable encounters I've been in have involved powerful enemies that weren't the ones in charge. A simple fix to your problem is to allow/cause the party to target the boss only after the weaker (but still powerful) minions are defeated.
This can be done by placing restricted ...
Absent Extenuating Circumstances, Yes
According to Kobold Fight Club, which gets its information from the DMG, a party of two level 4 characters would consider 250xp worth of monsters Easy, 500xp Medium, 750xp Hard, and 1000xp Deadly. Furthermore, they should not encounter more than 3400xp worth of monsters in a single day (between Long Rests).
A CR 12 ...
Build a criminal network as your BBEG
I have had success over the years in using the Organized Crime family model (an example is mentioned here) to provide scalable challenges to parties of good, neutral, and evil/chaotic alignments. The emphasis is on rivalry as the tension builder between your PCs and their nemesis.
What are the advantages of doing ...
Congratulate your player on solving a problem without fighting. Really. It does not often happen in FRPGs and yet even ancient cultures managed to avoid fighting most of the time.
Talk to the group about whether they would like you to craft encounters where not-murdering-everyone was a viable solution.
Incidentally this seems very much the way a bard ...
Hit your fighter in the NADs1
AC isn't the only way to cause damage to a fighter.
Utilize creatures that force saving throws. High AC won't save you from a fireball
Give them hard choices. AC won't save you when you have to choose between helping the rogue or the wizard…and you only have time to help one
Talk to your players. Let them know the high AC is ...
Group Checks, Success with a Cost, and Skill Challenges
These three concepts are meant to be used in tandem, but can be cherry picked to suit your style of play. I have the most experience using them together.
This is pretty simple. Half the group has to pass at doing some thing. In the examples you provided, this represents the party ...
Combat as a mechanical resource
If you are using XP progression (which is the default way to play, according to the PHB) and you want to give your player characters some catching-up, then you could use an easy combat encounter for this purpose. Similarly, you could give the party items as a reward from such encounters.
Combat as a pacing tool
Carcer and I can independently verify that all your math checks out.
That said, take a deep breath. I doubt this question was prompted by your lack of faith in your own math and reading skills, but instead by your DM's insistence that this encounter wasn't deadly, and this much bold formatting and all-caps in a question (prior to style edits at least) makes ...
I used to play a game that was fun and exciting: you rolled a dice and depending on the result moved up some ladders or slid down some snakes and the first one to the top won, its name escapes me for the moment. It was thrilling and intriguing and then I turned 5 and realized it was no fun at all because I had no agency.
My definition of agency is:
First things first, you have to identify the problem.
If his AC is too high, you could review his sheet and check if he didn't
calculate something wrong (it happens more often than you could believe).
If he built his character around that much defense, he will have other flaws, but lets take a look at the information you gave us:
For a short comparison: ...
If your boss can cast Animate Objects, that’s already factored into the boss’s own CR. Spells are always part of the caster’s CR, including spells that can add new opponents, whether by summoning, conjuring, creating, or animating them.
Since XP awards are derived from CR and the animated statues don’t contribute, they don’t affect XP awards.
Sometimes a skill-focussed player can bypass entire obstacles with that skill. This is a shining moment for them (which you don't want to step on), but boring for the rest of the group.
The general principle I'd follow here is "Yes, but...", useful throughout GMing: Don't say no, but do say what obstacles arise as a result.
First, take a look at the ...
You are screwed
Sorry to be the bringer of bad news, but you've painted yourself into a corner here. You told your players that magic users are "generally welcome, but some places hate them". This to me sounds similar to how you'd expect to be treated when you play a half-orc or a monster race. Some places will hate you, but overall you should be fine.
As you're aware, the DMG's guidelines for building encounters are designed for parties of three to five characters:
The preceding guidelines assume that you have a party
consisting of three to five adventurers. (page 83)
Many of the published adventures contain recommendations:
Lost Mine of Phandelver: "...adventure for four to five characters..."
Neither and both.
Unlike in Previous editions, CR isn't used to directly create combat encounters, instead the XP values of the creatures ( which is tied directly to CR) is used to determine how many creatures can be used in the encounter. Instead, CR tells you the upper maximum difficulty of the monster, assuming a party of 4. One of the reasons why they ...
Alternative Solution: The enemies don't matter
You said that
My goal is to make coming out of the dungeon a frantic race against time.
Therefore, it doesn't really matter how many skeletons you pitch against them, as long as it feels like a battle they can't win. This is a place where the numbers are irrelevant and it's more about creating a compelling ...
Don't waste a valuable resource
These are 5th level characters. As pawns they are well above the norm.
When they confide in the Spymaster, rather than kill them, she uses them. Turns her considerable skills and resources into framing someone else. Someone who is a threat to her. Maybe a minor noble she can't kill directly. She helps the party find the ...
You should scale the encounters to increase the difficulty for your party.
The ability to scale encounters to your party is very important when you do not intend on following a campaign to the T. The DMG actually clarifies how to do this for your party on page 82. Where it tells you how to gauge your party's difficulty rating by XP values. Within this ...