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 ...
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 ...
There are a number of answers:
I speak as a current player of a 5e warlock, who has 20 cha and agonizing blast, and who does not at all feel like an overwhelmingly dominant force on the battlefield. I manage to do my part, but "trivialize" would not describe.
Melee troops getting up close and personal with the warlock. If you're standing next to an enemy,...
I have this exact issue in a game I'm running (and the warlock can now fly, making him even more effective!). My strategies are still evolving, but I broadly have two categories I use to provide challenges for him: hard counters and soft counters.
Of those, I strongly prefer the soft counters.
Soft counters are anything which frustrates or ...
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 ...
My answer is not a long-term strategy, but it would be effective for one or two encounters and has not been suggested in the current answers. Your current issue seems to be that the spell-casters are too effective, in that they have a single small set of spells which they use to devastating effect. There exists a monster designed precisely to deal with such ...
I think the other answers saying,
"Yes, this is 8× more deadly than they'd usually face" and
"Try running the encounter yourself and see what happens"
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 ...
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 ...
The Generals of Hell Have Taken Notice of You
From: High Command
To: All Military Forces
Subject: Spellcasters Interfering with Plans
In response to reports from underlings about adventurer's with, dare I
say, fiendish effectiveness against our scouts and sorties, we are
forced to take extreme measures. ...
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. ...
There is no established lower bound, so that's up to the GM
The boggle's Dimensional Rift trait (Volo's Guide to Monsters, p. 128) simply says:
As a bonus action, the boggle can create an invisible and immobile rift within an opening or frame it can see within 5 feet of it, provided that the space is no bigger than 10 feet on any side.
...and the ...