This is something which I'm contemplating doing as an experiment, and I wonder what others might think of this.

My idea is that I take the monster stat blocks in the books as a baseline, but then have these monsters take a level or two from a class.

So, for example: Goblins.

A basic goblin is CR 1/4, so they basically disappear in later levels unless they are served up by the boatload. But what happens if I take that goblin and "Level up" and "Multiclass" them into a player class. For example, a goblin Barbarian.

The Goblin keeps everything it has already, and then gains the level 1 Barbarian features and health dice. This suddenly becomes a bit more interesting to fight, and somewhat tougher!

So, I am contemplating how much of a CR change a Goblin taking 1 level in any class would need. I'm guessing it would be closer to CR1, due to higher HP and additional abilities.

The goal being that players seeing a trio of Goblins would have no idea what to expect, which will remove meta knowledge of monsters. It'll also allow low CR monsters to play a part when players are a bit more powerful.

Has anyone tried this? I wouldn't go more than 1 or 2 levels as after that there's probably far better options from more powerful monsters!

Personally, I find it’s very nice on both sides of the DM screen to have the players know what to expect. If any given creature can pull any attack out of nowhere, it’s impossible to strategize or plan beforehand, and it gets harder to tell how strong the enemies are, so you don’t know how many resources you might need.

When it comes to player levels, be careful, especially if you are mostly using player levels. DND isn’t balanced for PvP. Players are glass cannons compared to most monsters, so be careful when using player classes.

I’d recommend just increasing the health, and pick-and-choosing features you want them to have. Then recalculate the CR using the CR calculator tool in the DMG.

I have warned my players not to assume, before meeting a monster for the first time, that monsters will have the abilities it says in their stat-block in MM or one of the other books. However, once the monster has been met in character, I generally don't change it around.

I think it's a good idea and a good way to spice up boring old creature stats, but if you're going to do more than one or two levels maybe consider using the Sidekick rules instead? They seem a little simpler to manage.

Do it!

But... Telegraph to the PCs that something is different. The monsters look bigger than what they have faced before, or are wearing different clothes, or are a different color. Perhaps they are cleaner or dirtier, or wear jewelry. Describe them the same way you would normally but add some specific detail to highlight that they are different. Play with the characteristics to give variety. If you do this to one monster per session for 3 sessions the Players will be more cautious and start listening closer to what you say.

For example: The PCs encounter 5 skeletons, two of which are red in color. They behave the same and have stats as the other skeletons but when they attack they do +1d4 fire damage and when they die they explode for 2d6 fire or piercing damage. Perhaps there are blue (cold) and green (poison/acid) varieties as well. Plot hook: Find this necromancer!

Quote from WilsonJ >>

Do it!

But... Telegraph to the PCs that something is different.

 I think I would do something like that. 

Even better if somehow there is a survivor so it becomes a return enemy.

I mean I am not against it, but it seems like it would be a lot more work on your end to create a balanced creature.

You could just re-skin them. Use orcs, but say they’re goblins. Then you know what the suggested cr is for the creature (for what that’s worth). And I’d agree with the others that you say something like, this particular goblin tribe looks unusually beefy. Then you can always go back to standard goblins again, to keep the players on their toes.

It is a great idea to customize things in your game, so that your more experienced players won't grow bored or complacent. Most of us have read all the books. We know what each monster can do and how to kill it. So at that point the game simply becomes an exercise in dice rolling and hit point attrition. That's not fun. I like to be surprised. I like to be challenged. 

That being said, I think adding character class levels to monsters will simply make your job harder than it needs to be. I think it would be easier to simply add one random (or chosen) ability to a monster. For example... maybe a tribe of goblins is led by a spellcaster, so each goblin knows how to cast one cantrip. Goblins shooting Fire Bolts is a problem. Or you could simply make a few goblins tougher: add a few HP, and give them 2 attacks per round instead of 1. Or maybe a higher level party is hunting a troll. They have read the troll stat block. But they discover THIS troll has the displacer ability. 

There are tons of ways to customize things. The trick is to find a happy balance between keeping things new and exciting for your players, and not bogging yourself down with hours upon hours of added prep time and game management. 

I do think, by the way, it is totally fair to have tougher variants of monsters. The MM and other guides already have this... You can have a Goblin Sergeant and a Goblin Captain, who are tougher variants than the regular gobbo. Same with many other types of monsters. There is precedent for many types of creatures, and there is no reason you can't add your own variants.

Just make sure that like the guides and MM, you describe them as being different from the regular ones so the players know something is up.

One easy way to make tougher versions of normal is to go to the NPC stat blocks at the end of the monster manual. The DMG actually has a table that tells you how to convert these stat blocks to the most commonly found humanoid races.

For example you could make a goblin druid by getting the druid statblock, increasing dex by 2, reducing str by 2, Nimble Escape; Small size; darkvision 60 ft.; speaks Common and Goblin.

A lot of people I speak to don't realise this table exists, in fact the table also tells you what the statblock for a zombie version of a creature would be. +1 Str, +2 Con, −6 Int, −4 Wis, −4 Cha. Undead Fortitude; immune to poison damage; can’t be poisoned, darkvision 60 ft.; can’t speak but understands the languages it knew in life

Quote from AnzioFaro >>

That being said, I think adding character class levels to monsters will simply make your job harder than it needs to be. I think it would be easier to simply add one random (or chosen) ability to a monster. For example... maybe a tribe of goblins is led by a spellcaster, so each goblin knows how to cast one cantrip. Goblins shooting Fire Bolts is a problem. Or you could simply make a few goblins tougher: add a few HP, and give them 2 attacks per round instead of 1. Or maybe a higher level party is hunting a troll. They have read the troll stat block. But they discover THIS troll has the displacer ability. 

 This is what I like to do. You don't need to stay within the constraints of PC classes or existing monster abilities or anything. If you want to have a bugbear that attacks by throwing goblins at the party, just write it up as a feature and spitball the damage and defenses based on an existing monster of the CR you are aiming for.

Obviously you don't want to invalidate past experiences, but describing an exceptional individual or group of monsters is a lot of fun and really builds excitement once your players know that these guys are a step above what they've encountered up to this point. These fights are almost guaranteed to stand out in the players' memories and I can't recommend it enough.

To bump something from CR 1/4 to CR 1/2 you generally want +40% hp and damage; to get to CR 1 you want +100%. For CR 1/8 and 1/4, adding a fighting style and a hit die will generally be sufficient, CR 1/2 may need slightly more.

Adding Superior Technique is a good way to give your monsters a bit more personality, though it does add complexity.

About half the monsters I use (and all of the boss monsters) are custom monsters. You can use D&D Beyond's Create Monster tool to do it, and if you base the creature on an existing one you shouldn't go too far wrong.

As others have said, show players that some monsters are different and give them visual clues. If the party has fought Goblins before, and somehow the Goblins all now have breath weapons, Ogre stats and can Rage, then how are they meant to plan for them? So just describe the goblins as being exceptionally large and bulky, that they carry weapons their small size shouldn't allow them to wield, and that they have a manic look in their eyes. After all, there's a big difference between a poor old Bear that has been kept starved in a cage for a year and a fat, salmon-fed Grizzly.

Two comments. First, I very often tweak monsters and minimize the CR calculation by not having any problem nerfing or buffing on the fly as the circumstances suggest. Second,  I will alter how tactically efficient I play them based on their motivation (goals, strengths, flaws). This can radically alter an encounter. 

I know it’s heresy to some, but this a key difference between material prepared for mass consumption and what you present at the table. If you are tailoring things to your party it doesn’t matter if you do it ahead of time or not as long as it doesn’t come off as arbitrary in the moment to the party.

Maybe I make that cool action a one time per encounter ability after realizing it’s way more effective than I thought. I might provide access to healing or throw in reinforcements. I might intentionally make a tactical error or on the flip side I might show I mean business by attacking an unconscious PC.

You should just do it. Not a single creature or monster or whatever has stuck to their stats on the monster manual. Every one of them had been changed.

But why? Easy! A lot of players see a creature. Like a goblin. They know goblins and what they can do. So they see a fight they know how to counter. Until a goblin casts banish on a player. Or goes into a rage. Or they use pack tactics. That's how I keep my players on their toes. I can say they are fighting goblins. They can look up the stats online to meta game. What they didn't see coming were things that are not in those stat blocks. 

I'll never use a pre-made stat block. And my players are both thankful and stressed about what could happen. They question every enemy. And that's fun. 

Quote from Greg >>

I know it’s heresy to some, but this a key difference between material prepared for mass consumption and what you present at the table. If you are tailoring things to your party it doesn’t matter if you do it ahead of time or not as long as it doesn’t come off as arbitrary in the moment to the party.

You do need to be careful here. I've played in a game where the monsters always happened to have the right tool for the situation, and it became apparent pretty quickly that the combats were being heavily adjusted in real time. It was done in the spirit of providing challenge and drama, but it was difficult to ignore and it prevented us from ever having a runaway victory - which I think is appropriate on occasion when you come up with a really good approach. Typically I try to limit my on-the-fly stuff to hit points and behavioral/tactical decisions for this reason.

It depends on the monster, so in my world Orcs, Goblins, Bugbears etc are part of a very involved complicated culture, they have been oppressed, suffered attempted genocide and had to leave their homeland, as a result they are more warlike and aggressive but, they have rogues, druids, fighters, barbarians, everything that all the other races already have. 

I will say however that you need to be careful about just porting stuff over, and, I only do it with "intelligent" creatures that have a culture and civilisation