Death Knights are, conceptually, among my favorite D&D monsters. While the Lich is arguably the more iconic ultimate undead bad guy, the Death Knight speaks to my love for a heavily-armored character - whether good or bad.
Of course, the most iconic Death Knight in pop culture isn't strictly undead: while Star Wars is more science fantasy than strict science fiction (though those lines are blurrier than you might think,) what it doesn't really have is the codified tropes of the fantasy genre (tropes that arguably got a lot stricter with the introduction of games like D&D). Still, all that being said, I think you'd be hard-pressed to find a more fitting Death Knight than Darth Vader - he's a powerful magical warrior who betrayed his moral philosophy in pursuit of power, and is now a scarred and broken echo of the vibrant man he once was, and to top it all off, he wears a facemask that is designed to look like a skull.
(I'll concede here that the Ringwraiths of Lord of the Rings might be a more direct and pretty spot-on example of Death Knights, but none has the personality that Darth Vader projects.)
Anyway, I've written about Death Knights before (probably several times before,) so we won't rehash all of that. Instead, I wanted to talk very specifically about encounter-building.
One of the great corrections that the 2025 Monster Manual made to the Death Knight was to make it a legendary creature. Shockingly, the 2014 version was not, and even if the official art depicted the iconic Lord Soth (D&D's most iconic Death Knight,) it could potentially be dealt with by a single unfortunate saving throw.
2025 not only gives us a legendary version of the CR 17 prime Death Knight, but it also gives us a less-powerful (but still quite powerful) CR 11 Death Knight Aspirant, which is actually low enough to serve as an elite minion at high levels.
The stat blocks are very similar, and so, while it would make some thematic sense that an Aspirant could be part of a Death Knight encounter, I'd actually advise against it. Instead, an Aspirant could serve as the focal point (or even sole foe) of a lower-level, non-climactic encounter, but using them with a true Death Knight would likely make things feel a bit redundant.
Let's consider what the Death Knight (the full version and the aspirant) bring to the table:
Both are primarily melee combatants, with three Dread Blade attacks and a fairly high AC that is more or less justified by the equipment they wear - plate armor and perhaps a shield. Granted, their Dread Blade does 2d6 slashing with some necrotic on top, so it's up to you whether this is some souped-up Longsword with a Shield or if they're wielding a Greatsword and are wearing +2 Plate. None is included as "equipment," meaning the party can only loot what you allow them to off of them, so it doesn't really matter (you could justify this by suggesting that the weapon's power is due to their unholy strength, and the armor might not be salvageable because it's intertwined with their undead physiology.
That said, +2 plate is not a totally unreasonable piece of loot to find on a CR 17 monster.
But let's get back to encounter design.
Aside from their melee attacks, both Death Knights have a recharge ability: Hellfire Orb.
Now, this is an important thing to note. Hellfire Orb hits for a gigantic amount of damage. The lesser, Apsirant's version of it, does 42 damage in a fireball-sized sphere.
For this reason, I'd caution against using multiple Death Knights, even the non-legendary Aspirants, because several monsters likely at the same initiative all tossing these on turn one might be more of a nova than you actually want your monsters to put out (I learned this lesson very early on when I had a level 7 or so party fighting a bunch of black shadow dragon wyrmlings, and realized that breath weapons are not really designed to go off like 6 at a time).
One option to mitigate this potential damage burst is to simply start their fight with the ability requiring a recharge - they can still use it on turn one if they roll a 5 or 6, but otherwise there should be some staggering.
Still, here's another reason to limit how many you use: Death Knights, even Aspirants, should be rare.
This is one of the challenges in D&D, especially at high levels. High-level players can handle really serious threats. But never forget that a Commoner represents a normal person - you, reading this, in a D&D world, would probably have around 4 HP. And that means that even a lowly skeletal soldier should feel like an absolutely deadly threat to most people, even if the heroes of our story can shrug off a shortbow arrow with little worry.
By this logic, then, a single Death Knight Aspirant could lay waste to the entire garrison of a fairly powerful nobleman. The only hope that a group of Guards and Veteran Infantry have against one is numbers, and the Hellfire Orb really punishes them for trying to overwhelm the Death Knight by hiring lots of guards.
In a fantasy world where people are aware of such threats, though, perhaps a more powerful person, like a king, would have more elite warriors who could actually stand against a monster such as this (the players being the optimal example).
A Death Knight on their own is a menace. But they're also not built to be fought on their own.
Marshal Undead, found in both stat blocks, projects a 60-foot emanation that grants advantage on attack rolls and saving throws to all other undead of the death knight's choice. A 60-foot emanation is enormous. Roll20's standard map size (and I think pretty close to my Chessex vinyl mat I use for in-person games) is 25x25 squares. If each square is 5 feet, that's a width of 125 feet, which means that a Death Knight in the center of this map covers essentially the entire map with it (ok, I suppose that the corners might not get as much coverage).
The next question, though, is how many monsters you feel comfortable running.
Because the lower of monster CRs is 11, a 5-player party is not really likely to face one of these until level 7 at the earliest. By level 7, the number of rank-and-file skeletons you need to equal a single player in terms of encounter budget for a low-difficulty encounter is 15.
Here's the thing about fights with tons of low-level monsters: they can be fun, but they can also be interminable. Practically speaking, they can also serve as just a yes-or-no "fireball check." If you have 30 skeletons firing their shortbows from a castle courtyard, a single Fireball will probably take out any it can hit, which could be a big portion of them, meaning a Sorcerer or Wizard (or Light Cleric) could handle the problem very quick. If not, though, like if your party's damage dealers are martial classes, even if you had enough damage to kill a skeleton on every attack, your level 7 or 8 Fighter, Barbarian, Paladin, or whatever, is still going to be able to take down at most, like, 1/15th of them each round.
To be clear: narratively, the idea of a Death Knight leading an entire platoon of skeletal warriors makes total sense, and gives you a really cool image. But you're either going to spend minutes setting them all up on the board only for the Sorcerer to delete them with a fireball, or you're going to have your tense encounter devolve into an utter slog.
The good news is that we have far more options for minions than CR 1/4 Skeletons.
Even just within the Skeleton category, we have things like Minotaur Skeletons, which are CR 2, and thus can account for 9 times the xp budget of a standard skeletal minion (the stat block is also probably very easy to re-skin).
Likewise, the new Flaming Skeleton not only gives us an even tougher skeleton to work with, but they're even immune to fire, which might push players to have to take them on individually (which will be less tedious because you won't have as many of them thanks to their higher CR/XP value).
More or less any undead monsters could serve as minions for a Death Knight, though I tend to think, on an aesthetic level, that corporeal undead, and especially of the skeletal variety, seem the most appropriate.
The Death Knight does have some ranged capabilities, but they're either limited in use or require a recharge. Thus, I think minions capable of ranged attacks make a lot of sense. The Marshal Undead aura is large enough that they can still benefit from it even if the Death Knight enters the melee, but a mix is probably appropriate, perhaps with some tougher bruisers helping the Death Knight hold the line and protect the ranged monsters.
Mobility is a consideration as well: the Death Knight, being a Knight, is often depicted riding a steed (again, we can look to the Nazgul from Lord of the Rings, first with their iconic black horses and then on their fel beasts, the best stat block for which I think is probably the Gloomstalker from Explorer's Guide to Wildemount - not to be confused with the Ranger subclass).
The skeletal horse is the obvious choice here for a mount. But there's a slight problem: a Warhorse Skeleton has only 22 HP and vulnerability to bludgeoning damage. By the time you're fighting a Death Knight, the party's Monk could come in and pulverize such a mount in just a couple attacks, which would then presumably send our menacing monster prone on the ground amidst the shards of their steed.
It's kind of the same problem we find with using standard skeletons - the CR imbalance creates and inappropriately weak monster for our encounter.
So, let's consider some upgrades. The most obvious one, to me, is a Nightmare. With 68 HP, players will need to really commit to take one of these down. Furthermore, Nightmares can fly, which could allow the Death Knight to stay in the fight longer (perhaps even remaining airborne and just waiting to use their Hellfire Orbs when they come off cooldown - this reminds me a lot of the Headless Horseman tossing flaming pumpkins in World of Warcraft's annual "Hallow's End" event. Yes, there's a separate stat block for a Dullahan, but they're very closely related monsters).
A Nightmare is built to be a mount, with a teleport option that takes its rider and also a conferred fire resistance (the Nightmare is fully immune).
If we want to get super-epic, a Dracolich could work as a Death Knight's mount, but we do run into the problem of having multiple legendary monsters in a single fight. Dracoliches are primary villains in their own right.
All that being said, a Death Knight on foot is not unthinkable. This works especially well if the encounter involves the Death Knight attacking some bastion of safety - an allied noble's castle, or a temple of some sort. Fundamentally, a Death Knight is only as large as the person they were in life (meaning they can even be Small sized, if they had been a Halfling or Gnome, for instance).
Now, mindless undead minions like the aforementioned skeletons (again, I'd favor Minotaur and Flaming varieties) work fine, but Death Knights are also among the intelligent undead. While their state of undeath might cause them to pursue their goals in a compulsive manner, they're of reasonable intelligence and can speak.
And thus, you could also imagine that their minions might not be so dumb either. Wights work quite well here - already a kind of intelligent undead warrior. Revenants are also a possibility, though I think that the lore here is a bit distinct - you could re-skin a revenant, of course.
All right: we've figured out some ways to make the encounter a challenge. The stat block does a lot of that already, and some of our minion choices, like a Nightmare mount and Flaming Skeletons, undermine the most obvious go-to strategies for taking down the Death Knight's squadron.
How, then, can we add things that give the players an edge?
Given the militaristic nature of this undead foe, I think that our ideal encounter with a Death Knight is a defensive one - something like a siege. Without a flying mount like a Nightmare, the Death Knight is gong to be somewhat limited in their mobility. Giving the players an opportunity to hold the Death Knight at bay for a round or two could be very good (though only if the encounter is tuned pretty unforgivingly - if the party has the power to take the Death Knight down before it gets into melee with anyone, that's going to feel anticlimactic).
Amble opportunities to take cover would be great, especially if they can stay out of line of sight of some of the Flaming Skeletons. Also, that Hellfire Orb is going to hurt quite a lot even if people succeed on their saving throws (unless they have Evasion,) so I would say give the party plenty of opportunities to take advantage of cover and be more likely to succeed on their saves.
Turn Undead will certainly have the potential to undermine the difficulty of the encounter, but that's ok: sometimes, as a DM, you want to, as one of my players put it, "Shoot the Monk," and play directly into their strengths. Marshal Undead will give your minions a greater chance to succeed against the feature, but if the Cleric manages to separate out some of the minions, it'll make them feel like this is exactly what a Cleric is built to do.
One of the fun things about a Death Knight is that they might not be so obviously undead. Chances are, especially for the legendary version, you'll want to build them up as an important NPC. Indeed, a Death Knight could be the big bad of an entire campaign (albeit probably one that doesn't go too far beyond tier 2).
You can imagine, though, that given that they're encased in full armor, they might be mistaken for a rogue knight. Some "black knight" who has been terrorizing the land. Even better, the Death Knight might even show up initially not as a villain, but presenting themselves as some neutral or even friendly party. The game is given up quite immediately if the party uses something like Detect Evil and Good or Divine Sense, and while a Lich under some kind of illusion might use a spell like Nystul's Magic Aura to fool such detection, a Death Knight's paladin suite of abilities is unlikely to have access to that, so unless they're working with a Lich or some other powerful wizard, they're probably not going to be able to prevent such detection - so just be wary that if the players are remotely suspicious, this ruse might not last.
That said, you can still get a lot of drama out of this: say the party is in the court of our allied nobleman, and something about this knight (maybe the fact that they never take off their helmet) tips the party off that something's fishy. The Paladin might be fully aware of the Death Knight's undead nature, but convincing their ally that this is the case is another story. Perhaps this knight has served the nobleman for decades, and so the nobleman assumes that no one who has given such faithful service could truly be evil (the Death Knight, of course, thinks nothing of serving for decades to get what they want). On top of that, the Death Knight is fairly charismatic, and might be able to persuade the ally that there's some reasonable explanation - even framing their undeath as a curse that they have long sought to cure, rather than a reflection of their own corruption (you're also free to make a non-evil Death Knight under such a curse).
However, the truly dangerous part of this scenario is what happens when that accusation lands. In any story of intrigue, the protagonists need to be very careful about when and how they reveal the secrets they've discovered. If your level 4 party has just proven themselves to the local earl by defeating the local coven of Green Hags, only to meet this loyal knight of the duke's, they don't want to give a death knight a reason to kill them, because it's very likely the death knight would succeed in doing so.
Let's talk spells:
Both Death Knights have access to a couple of spells. The Legendary version gets some others that they can use as legendary actions.
Each has Phantom Steed - this actually works out pretty well, because while it's very easy to take a Phantom Steed out, dong so won't send the Death Knight prone, so it can be a tactical decision to spare a couple of attacks to reduce their speed effectively from 100 to 30. If you don't want to add the XP of a Nightmare to an encounter, this is a good option to let them at least start off on a mount (while they can cast Phantom Steed at will, it does have a minute casting time).
True Death Knights also have Command at will, but I'd recommend primarily using this as part of the associated legendary action - Command is a great 1st level spell, but you don't want to spend your entire turn casting a 1st level spell.
Destructive Wave can be pretty powerful, dealing an average of 35 damage on a failed save or 17 on a success. Now, the Death Knight's melee attacks land for an average of 25 damage per hit, and they get three attacks, meaning that you're going to do far more damage to a single target with the Dread Blade. Even with two targets to hit, you're going to probably do more damage with the Dread Blade, so try to A: save this for when you can hit three players or more. Then, consider the following: 35 damage when the party's all fully rested and at full health is scary but not going to take anyone down. 35 damage well into a battle, when the Cleric just ran it to get the Fighter up with a 2nd level cure wounds is a lot scarier.
Hellfire Orb is there to get the party on their back foot - blasting for a massive 70 damage, you can take them from full health to bloodied. But it's also got no friendly-fire protection, so this is going to be an ability that the Death Knight uses on turn one, before their minions have closed in on the party. Destructive Wave only does half as much damage, but it doesn't hit the Death Knight's allies, so they can use it even in a dense melee.
Finally, Dispel Magic is some cool utility, but you want to be really sure that it's worth it to spend an entire turn casting this spell. If the party has somehow trapped the Death Knight in a Magic Circle or the like, this might be the move. But I wouldn't use this on something like an Aura of Vitality - the Death Knight might be able to take the character concentrating on that spell down instead. This spell can be used outside of combat to dramatic effect, such as my dispelling a Hallow spell keeping them and their minions out of some place (note that as a 5th level spell, the Death Knight only has a 50% chance on succeeding on their Charisma check to dispel Hallow).
As a final note, don't be afraid to throw a Death Knight at a party early and often. Like the Lich, Vampire, and Mummy Lord (and basically all fiends,) there's a codified "get out of jail free card," at least for the legendary variety, where a Death Knight can always come back as long as they haven't atoned for their evil. The Aspirant, admittedly, does not have this in their stat block, but you could easily make an exception if you want recurring villains.
The Death Knight could thus be a recurring threat, and the encounters with them could evolve even while using the same stat block. Perhaps the first time the party encounters them, they're a solo monster. But as they climb in level, the Death Knight's encounters become more challenging (or rather, remain level-appropriate) by having them accompanied by more minions and more powerful minions.