I'm about to lead a raid into an orc camp for a party of two characters. Both are level 4, one is a bard and the other is a barbarian/warlock multiclass.
I found a CR12 homebrew monster, the Orc Druid Gatekeeper (shared by /u/FreeWolf on the /r/UnearthedArcana subreddit); it looks fun but I don't want it to kill them. I can't find anything that says firmly what it should be. I'm a new DM and don't want this to overwhelm them.
Is this CR12 monster likely to be too difficult for my players?
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 monster is worth 8,500 xp, over eight times the level of a Deadly encounter. A CR 12 monster would almost certainly kill your players within a round or two, unless they were exceptionally clever or lucky, in which case they might be able to flee.
Note that this assumes a) that the homebrew is indeed CR 12 (many homebrews' CRs are inaccurate, since it is difficult to calculate without extensive playtesting) and b) that the fight is more or less fair. Many circumstances, such as environment or the presence of allies on either side, could influence the difficulty of a fight. However, if circumstances influenced a fight so heavily that your party was able to defeat a CR 12 monster, the players' contributions would probably feel negligible.
Kobold Fight Club (linked above) is an excellent tool for planning and balancing 5e encounters.
If you have a copy of their character sheets, run the encounter by yourself a few times playing both sides. You should get a decent idea about how outmatched the PCs might be.
Most likely, Yes
A creatures' CR is roughly the character level for a 4-person group to have a decent challenge.
A group of 2 level 4 players would be around CR 3 themselves... You are putting your players against something that can hold its own against 4 level 12's, and is deadly to 4 level 10's.
Now, you can of course make up reasone why this creature is weaker than usual, or you can give your players an obvious advantage
Looking at it from a "CR budget" stand point is fine and dandy, but as we are all aware CR isn't always the most accurate depiction of difficulty (though admittedly this is much less pronounced at low levels).
Look at it this way.
First, go to Kobold Fight Club and isolate CR12 monsters.
Then remove everything with a legendary action and legendary resistances, because the outcome of those fights will be clear. Barring some extraordinary circumstances, the monster with legendary actions will stomp the L4s. For example, a pair of L4s aren't about to reasonably fight an Int 19 Ki-Rin that can throw out a Holy Word and then take 2 legendary actions to either move 60ft, or throw out free 4d8 damage sacred flames.
A level 4 Barbarian with 16 Constitution and fixed stats has 45 HP. That's probably going to be about as good as it gets for HP at level 4. A chain lightning or an up-cast Cone of Cold will probably do that much damage. Fireball damage is comparable. Not to mention that, if the monster is a high-level caster, it's probably smart enough not to reasonably let a bunch of L4s get the upper hand on it. So, you can also remove all of the casters with access to level 5+ AoEs.
You are not left with many options. I suggest that the easiest of these is something like the Gray Render or a Frost Giant Everlasting One, either of which is likely to drop a PC each round. I would say that the Gray Render is likely the easier of the two, so let's go with that.
The Gray Render has 19 AC. It gets 3 attacks, 2 of which prone at DC16, doing something like 54 damage/round. However, it's a large dumb brute with a 30' move speed and can probably be CC'd relatively easily via an intelligence or Wisdom save (i.e. - Phantasmal Force) and worn down over time at range. It's conceivable that something like a duo of casters could beat it under the right circumstances. Similarly, a duo of optimized ranged characters (say, L4 Variant Human Rangers with Sharpshooter and crossbow expert, or PCs with a fly speed) could also kite it and whittle it down if they fought it in an open field.
Apart from this, the PCs' advantage would have to be extreme (i.e. - 2 barbarians inside an cage inside an anti-magic field vs an Archmage), in which case the monster probably wouldn't be fairly CR'd at 12.
I think the other answers saying,
cover the reasons not to use the CR12 creature.
Also consider, look at a single attack from it. Would that be enough to kill one of your player characters? Are you ok with that? If not, that's enough warning all by itself. One too-strong monster can upset everything by unexpectedly killing a character even if it's theoretically not too strong (e.g. a CR12 creature against twenty level 4 characters).
What can you do? Well, since it's a homebrew monster, no-one knows how strong it's supposed to be. Can you massively drop its attacks and HP until it's roughly equivalent to an existing CR3 monster? (Either "that's just what it's like in this world," or "it's a cub.") Or, if you don't have that much experience gauging what's reasonable, take the stats of an existing CR3 monster and replace one or two of its abilities with an interesting ability from this new monster to create a simplified version of it.
In particular, think through a few version of the first few rounds of combat, what's likely to happen, which give the experience you're hoping for, and focus on those, rather than the stat block as a whole.
There is no "too high" The DMG pg 81-85 is devoted to encounter building. From a practical standpoint this is a killer encounter that the PCs will not walk away from unless they can outsmart the baddie or flee. In general according to the DMG a Party of 2 level 4 players would have a CR of 2. This can be changed based on your players skill and capabilities. Another option is you can change the stats of the CR12 creature to better fit the party level as noted on DMG pg 273-283.
Mathematically, there are two dimensions to encounter building - monster Challenge Rating and encounter XP budget. CR tells you whether a monster is a fair challenge, XP tells you whether an encounter is a fair challenge.
Challenge Rating is defined as the lowest level at which the monster is a fair challenge. Since this monster's CR (12) is [much] greater than the characters' levels (4), it is absolutely not a fair challenge.
Lots of good answers, I'm going to go in a different direction and say:
Provided, you can think quickly, roll behind a screen and are OK with blatantly lying to your players.
Here's how you can do it.
You're the GM, you have all the knowledge and all the power. You can make this fight go exactly the way you want. It can crit the barbarian in the first round and bring him to zero in one hit. Your players will be scared to death, but subsequent, non-critical hits can be more reasonable. The monster can be a pansy who flees as soon as it sees your players are standing to fight. You can kill a character on purpose if you want. Resurrection quests can be fun. Turn the battle into a dramatic story, and let your players think it's a battle of tactics and dice.
Not everyone likes this type of play. Some people think fudging dice rolls ruins the game. Some players say, "If you're not going to follow the rules, then why do even have rules?" You have to know your players and know what will work for them.
