Two caveats:
First off, the campaign I'm currently running is at level 19. The players have fought the Tarrasque and killed it (half the party was unconscious at the end of the fight, so I kept my pride). There is nothing that is too high in terms of CR for them to face, so this isn't something I can do in this campaign anymore.
Second, I find it really hard, like, on an emotional level, to stray much from the guidance on combat encounter difficulty. I take great care to ensure that my players always face reasonable challenges, even if they might be tuned "hard," which, in the 5.5 DMG, does actually make for some pretty tough fights.
And yet...
So, I often see DMs, such as Brennan Lee Mulligan when running Critical Role's Campaign 4, or when he did Dimension 20's A Crown of Candy, talk about intentionally not balancing fights for the players' levels. The idea here is that the world sort of exists beyond the player characters, and it's up to them to ensure they survive it.
I think this makes a certain amount of sense, but notably it always seems to be invoked when the players are low-level. If I took this attitude toward my design for my level 19 party, even a plane as nightmarishly deadly as New Phyrexia (where they currently are) would still probably be trivially easy for them to manage except, possibly, when facing the Praetors (to be fair, I have basically designed the adventure as focusing mostly on boss fights with these characters, as I don't want them to be stuck at level 19 for more than a year, which I think was the case for levels 17 and 18).
But let's turn things around again:
Here's the thing: while there is resurrection magic in D&D (though notably, in both the campaigns I mentioned run by Mulligan, they're not allowed to use resurrection magic,) PC death should feel fair. In-world, of course, it need not feel fair - a tyrannical regime or deadly monster killing someone needn't feel fair, but it shouldn't feel unfair to the players at the table.
After all, as a DM, especially at lower levels, it's trivial to kill your PCs. Throw an ancient dragon at a level 3 party and they'll probably all die to the first breath weapon (even if they succeed on their saves).
However, there's a downside to making sure every combat encounter is fairly balanced: by the time you face a monster, you're ready to fight it, meaning that it never really gets to impose a sense of terror and dread.
I think my affection for Death Knights as monsters is well-documented here (I also love the WoW class). I think the most direct, clear example of a Death Knight in fantasy pop culture are the Ringwraiths from Lord of the Rings. They're undead, or the kind of Tolkien equivalent (they lived far, far longer than they should have, and their spirits remained even as their bodies withered away into nothing) and were basically noble lords who allowed this desire for immortality to corrupt them (I'm generally against the idea of death, disagreeing with the "death gives live meaning" rhetoric, but I sure don't want to live in a world in which asshole oligarchs like Elon Musk live forever).
Anyway, I also like Death Knights because they work really well as the kind of figure who could move about in society and not immediately draw attention to themselves. The Riders in Black who show up in the Shire asking about someone named Baggins certainly freak the hell out of the people they meet, but there's just enough plausible deniability to imagine that they're just, you know, creepy dudes and not ancient undead terrors.
So, the first really obvious way to use a high-level monster to scare a low-level party is just to use them as an NPC. Combat, of course, is just one of three pillars in D&D, and while it's the one that has the clearest and most extensive rules, it's not what the game needs to always center around.
Not every monster just kills everything it sees. Especially in the case of intelligent monsters, they probably have an agenda.
Let's concoct a scenario:
A, say, 2nd level party has gotten a quest to deal with a gang of bandits who robbed a caravan. It's a sort of secondary detail that the caravan they hit was bringing some artifacts from a ruined temple to a museum in the big city. The message the party gets is that this is just one of many robberies attributed to this particular gang, and it's a standard bounty quest.
The party shows up at the bandit's hideout, maybe an old ruined fort in the woods. When the party arrives, something's off. First, they find one of the bandits dead on the ground, a giant wound through their torso and into a blood-stained tree, and that the tree itself looks a bit withered where the weapon pierced it.
They arrive at the hideout, and find that there are bandits guarding the entrance, but they're not facing outward - they're distracted by something going on inside. Maybe this gives the party a chance to get the drop on them, or maybe they try to engage with them socially. The bandits, panicked, are probably hostile and still likely to fight. Maybe in the middle of a fight, a previously-unseen bandit emerges, seeming to be there to join the fight, but instead just screams at the top of their lungs and runs out into the woods.
In the middle of combat, there's an explosion: a Hellfire Orb has detonated elsewhere in the fort, and the fortress' roof is now burning with strange fire (maybe green, maybe blue - something otherworldly).
At this point, the bandits are probably going to just run for their lives. The party may consider doing so as well, and if that's the case, well, you've made your point.
But if the party's curiosity gets the better of their sense of self-preservation, they might go into the fort to try to recover the artifacts.
There, you can describe a scene of slaughter. If the bandit encounter was tuned to be a reasonable challenge, the party finds twice as many bandits all dead, the floor slick with blood. If the bandits have some bruisers with them, like an Ogre, maybe there's an Ogre who looks like they were slammed onto a shard of wood and impaled, making it very clear that a monster that would have been pretty serious for them is trivial to whatever has done this.
The party goes further into the fort. Maybe they find a survivor who is just hiding and can fill them in: that a knight in black armor they had never seen before came in and attacked without warning. Maybe they follow the trail of destruction down into the fort's basement, where the artifacts were being kept.
There, they behold the Death Knight (could be a Death Knight Aspirant - still high-enough CR to demolish a level 2 party) who is singularly focused on going through the artifacts. The Death Knight ignores them, and if they just stand and watch, the knight finally finds what they're looking for - an ancient stone key or something - and, if the party remains non-hostile, the Death Knight just pushes past them and leaves the fort.
Now, if you feel like you really want to hammer home just how deadly they are, here's another trick: the Death Knight isn't done with their slaughter yet - the bandits' leader, a Bandit Captain, is facing off with the Death Knight. The party might come in mid-battle, so the Bandit Captain has already taken a hit, and so they come in just as the Death Knight hits them with a lethal attack - make the attack roll, and roll the damage, and declare it all so the party knows precisely how much damage this guy does.
Let's consider various scenarios:
If the party tries to speak to the Death Knight, they might just remain silent, not interested in engaging with the party, because they're singularly focused on their mission, or perhaps provide only a gruff response.
If the party attacks the Death Knight, at this point, I think you have license to punish them. The first character to attack (maybe not in initiative, but the first one to suggest attacking) should be the Death Knight's target. Put them down - maybe don't coup-de-grace them, but consider that a Death Knight Aspirant hits with a single attack for 24 damage, which will likely drop most 2nd level characters in a single hit (even a Barbarian with a +3 Con will typically have 25 HP, so maybe two hits).
If the party leaves the Death Knight be after this, maybe only tending to the downed party member, the message is clearly delivered.
If the entire party attacks the Death Knight? Well, I think this is where you might need to get creative. On a certain level, this is a TPK the party has well and truly purchased with their actions. But if you don't want the entire campaign to be derailed, here's another option:
The Death Knight goes to strike, but pauses, looks up at the beams holding up the ceiling, the crackling fire raging above (remember that the roof was on fire?) and rather than striking the party down, simply pushes past them to leave the fort. Maybe one turn (not round) after they do this, the roof collapses, and while it's a lot of scary damage and a challenge to get out of the burning rubble, you make it a more reasonable thing for 2nd level players to escape. By the time they get out from under it, the Death Knight is gone.
Again, the ultimate fail state of D&D is a TPK. But I think particularly as players have become more interested in the narrative possibilities of the game, a TPK can be an unsatisfying conclusion to a story. It's certainly something you should talk about in a session zero (and later too).
Establishing villains early on is a pretty important way to set the stakes for a more narrative-based campaign. I think that the key to making the game feel reasonable, even if it's not "fair" in the strictest sense, is to really telegraph when the party is dealing with something beyond their means to defeat (and motivate them to get more powerful so that they can!)