In The Lord of the Rings, we arguably get the most iconic death knights in all of fantasy literature - the Nazgûl, which translates in the Black Speech of Mordor to "Ringwraiths." (Nazg is clearly the word for ring, given the poem on the One Ring, and I wonder if it's simply "ul," or possibly "gul" that means "wraith." The latter seems more likely to me given that it links it to the real Arabic "ghul," which derives from "ghala," meaning "to seize" and is, of course, the basis for the English word "ghoul.")
Death Knights have a certain potency because they turn the most straightforward fantasy hero - the knight in shining armor - on its head. In D&D and many other fantasy RPGs, the Death Knight is a mirror of the heroic Paladin class, and the backstory is often that of a paladin who has turned their back on goodness to instead serve evil. This betrayal places them in a cursed state of undeath, but they have many of the same capabilities they used as paladins, only turned backward - where Paladins are adept at fighting undeath and wield holy light, Death Knights are undead themselves, often commanding undead forces, and wield evil darkness.
Now, more recent editions of D&D have introduced nuance to the Paladin class. Though originally all paladins were lawful good in nature, today the paladin is more simply defined as someone who wields divine magic born out of their conviction and adherence to a sacred quest. They swear an oath, and their faith is rewarded with power to accomplish their quest.
The DMG presents an alternate subclass to the paladin: the Oathbreaker. While this is primarily intended for NPCs, a DM can also allow it as an option for players. Personally, I'd recommend DMs avoid building NPCs as full player characters with a bunch of class features, because that is an enormous thing to have to run along with everything else at the table.
But the Oathbreaker Paladin is clearly meant to represent someone on the path to becoming a death knight.
Returning to our example of the Nazgûl, the Ringwraiths are ultimately servants. Indeed, I think you could fairly well approximate the Lord of the Rings using nine Death Knights that are servants of a Lich (the One Ring being Sauron's phylactery). I might argue that Sauron is more accurately a fiend, which means that the Ring might be more akin to the amulets used by the Demon Lords to survive death even in the Abyss (my headcanon, until contradicted, is that Orcus taught Vecna how to make a phylactery based on this process of creating a demonic amulet, and that liches have borrowed demonic magic to become what they are). Of course, Sauron is 100% Lawful Evil, not Chaotic Evil (Morgoth is the latter) but you get the gist.
And, in a sense, I suppose this is why in the Monster Manual they are not legendary creatures - they serve some greater power that corrupted them.
That being said, one of my other major influences is the Warcraft series, and its most iconic villain is Arthas, the corrupted prince of Lordaeron, who eventually becomes the Lich King - the lord of the Undead Scourge. While later lore would reveal that the Lich King was at least intended to be a puppet for a god-like entity of death, the Lich King is like a Death Knight who commands an army of liches and other, lesser death knights along with countless undead in myriad forms. (Before he becomes the Lich King, Arthas undergoes the classic paladin-to-death knight transition). The point is, Arthas would most certainly be depicted as a legendary creature in D&D.
Indeed, I think that even as a minion of some greater evil, I still find it odd that the Death Knight is not a legendary creature. A Death Knight is no rank-and-file minion, but should fill the role of top lieutenant. The Ringwraiths are the most powerful of Sauron's weapons, and the Witch-King of Angmar is the closest he gets to a second-in-command (indeed, the Witch-King is more or less the world's most dangerous evil when Sauron is in hiding in the early parts of the Third Age, and successfully destroys Arnor, Gondor's sister-civilization in the north where Aragorn's ancestors ruled as king).
(As a note, we should also remember that most stories don't have "levels," so we shouldn't read too much into the fact that a single ranger scared off five Ringwraiths with a torch when considering the power level of our stat block.)
Now, in terms of creature design, Death Knights have a number of paladin spells. However, given the new focus on moving away from player-style spellcasting in recent designs, we can shift things around. Death Knights are already quite formidable in melee, so I don't think we need to add any kind of ranged spell attacks as part of their general rotation.
We're going to aim for our DK to be CR 17, like the 2014 Monster Manual version of it.
But let's also think a little more broadly about what we want out of a Death Knight.
First, we want them to be really deadly with their weapons. This is already reflected in the fact that the 2014 version deals an extra 4d8 necrotic damage with their weapons. With a normal attack, the death knight deals an average of 27 damage per hit, and gets to attack three times a round, which means a total average damage output (assuming all three hit) of 81 damage. The necrotic damage dealt also lets them get around the fact that their weapon isn't inherently magical (they have the Magic Weapon spell, but we're going to get rid of their spellcasting trait, except perhaps for some utility stuff like dispel magic). Player characters almost never care whether the damage is from a magical weapon, though, so especially with the necrotic damage in there, I don't think we need to worry much about making their weapons magical. We're going to take this into consideration when figuring out their Offensive Challenge Rating, or OCR.
But just being able to put out a lot of damage isn't really that exciting. We've got to think about what it means for these dread beings to act. One thing that I think is core to Tolkien's sense of power is the effect of being able to inspire emotions in others. In the books, the army of the dead that Aragorn brings with him to break the siege of Minas Tirith works primarily by terrifying the orcs, not so much killing them as driving them into disarray and breaking their lines, so that the defenders can pour in and mop up the invading army. Conversely, one of Gandalf's most potent traits is the ability to inspire those around him - he rarely goes around casting obvious magical spells, but instead fills the mortals around him with the courage and resolve to fight against the darkness.
As such, I think that this could play into our Death Knight design. And, because they're related to paladins, we're going to borrow something from a paladin subclass - no, not the oathbreaker, but the Oath of Conquest, which is another "evil" subclass - and one that I think is more lawful evil than the out-for-themselves Oathbreaker. Essentially, I think that they should have something similar to Conquering Presence, but that they can do continuously, and potentially fold in the Aura of Conquest, or something similar.
Now, fear effects are relatively easy to overcome, thanks to Paladins' Aura of Courage and things like the Halflings' Brave trait. But the DC here should be relatively high and coupling it with immobility could be quite powerful. Still, generally speaking I think a lot of fights wind up locking down in melee, and it becomes a race of trading blows. Mobility makes monsters a lot deadlier.
On the other hand, the Death Knight is not nimble and quick. If anything, the dread fear they inspire is only enhanced by their slow, plodding advance. How do we square these?
Borrowing a little from Blizzard, both Death Knights in World of Warcraft and Leoric, the Skeleton King in Heroes of the Storm (Leoric had it first) have an ability called Wraith Walk, which allows them to slip into a ghostly form and move quickly for a moment.
I think this does a good job of making them seem like the implacable foe that they tend to be portrayed as. Thus, I might give our Death Knight as bonus action to let them break grapples and restraints, and possibly even avoid opportunity attacks. While I'm tempted to simply make this additional movement, I don't think Death Knights should be particularly fast - they should be able to follow you slowly, like a slasher villain, and take all the time they need to catch up to you.
The only downside to this is that if it's a bonus action it's always on, rather than being a special moment where the party realizes just how much trouble they're in. We could limit this to a certain number of times a day, or make it a recharge ability (though recharge abilities I feel should be scary and deal extra damage).
Instead, this might be best done as a legendary action (though I'm finding myself drawn more and more to "extra, powerful reactions" as we saw with Vecna or the Eldritch Lich in favor over legendary actions,) which would probably mean giving them extra movement, but it might work the most elegantly - and costing two legendary actions would be a decent price to make sure you're not spamming this.
Going back to the Ringwraiths, when I first saw the Fellowship of the Ring, the screams they unleashed were very loud in the theater. You could feel how their screeching would fill someone with dread and cause a sort of involuntary shudder. This is, to be fair, very specific to Lord of the Rings (and perhaps very specific to Peter Jackson's films). But perhaps that could just be one way to skin some kind of dread wail effect - if we wanted to make it very scary, we could have it not only do psychic damage but also stun those who fail (if we want it to be a little less powerful, we could make this "fail by 5 or more and you're stunned.) I think stunning, especially for a melee monster (to speak nothing of paralyzing, which would probably be too powerful) is quite strong, so the damage itself of this ability should be only around the same as their regular melee attacks (though with the potential multiplier thanks to it hitting anyone nearby.)
I was going to get into a whole thing about raising the dead, but I think D&D has an unfortunate lack of mid-tier undead monsters that can be a decent threat as henchmen to a high-level undead monster - ideally you'd just use a ton of zombies and skeletons, but I think that unless you are using MCDM's excellent minion rules (they have a zombie minion in the latest free pdf, and a simple chart for scaling these up in level) that tends to just be tedious.
One thing I think is a sort of interesting question is whether a Death Knight counts as corporeal. Often, they're depicted as skeletal or zombie-like, and sometimes still more or less living but with a kind of pale complexion. Tolkien's Ringwraiths, though, were fully ghost-like. The Witch King is described as basically being a pair of floating red dots for eyes in an empty crown, their physical form fully passing over into the unseen world.
Their armor and weapons are physical, though, so the only reason I bring this up is how we're going to handle damage resistances and immunities. Ghosts, Wraiths, and other incorporeal undead in D&D tend to have full immunity to necrotic damage (and poison, but that's basically all undead) and resistance to a most common elemental damage types like acid, cold, fire, lightning, etc. (While few actually have vulnerability to radiant damage, their resistance to nearly everything else kind of effectively makes this true.)
The Monster Manual, interestingly, gives them immunity to necrotic and poison damage, but none of the other resistances (even nonmagical weapons work fine on them.) This feels like a reasonable balance.
As always, when using the DMG to figure out the CR of a monster, we run into the problem that the "Making a Monster" CR chart doesn't actually have any place for ACs of 20 or higher - absurdly, even a CR 30 monster on that chart is shown to have an AC of 19. So we have to kind of eyeball it.
I think my approach will simply to be not to touch the AC, HP, and immunities, and to try to have our new Death Knight put out the same damage as the old one. We'll be making it legendary, but I think the intent is not to account for that in the CR calculation - it's legendary, and thus supposed to be tougher.
Essentially, we're really just swapping out their spellcasting and possibly their Hellfire Orb for some new features.
Like the new spellcasters, we'll likely keep a few utility spells like Dispel Magic, but we won't rely on this for their damage output.
Just attacking with the longsword (one-handed, as they use a shield,) Death Knights put out 27 damage per attack, and get three longsword attacks, for a total of 81 damage per turn. So we'll basically keep that the same and treat that as a baseline for what we should expect them to put out.
Once per day, they get to shoot off a Hellfire Orb, which does an average of 70 damage (half fire, half necrotic) in a Fireball-sized sphere. Generally, when doing AoE stuff, we treat this as if it's hitting two targets, so essentially this is 140 damage. But this is a one-time thing, rather than something we might encounter multiple times.
Unless this is just because I haven't read any Dragonlance novels, the Hellfire Orb feels kind of off-brand. Paladins don't have Fireball. The DK does have the Destructive Wave spell, which is a paladin spell, which does half that (and has friendly-fire protection so they don't roast their ghoulish companions.
But given that we're leaning into the simplifying of stat blocks, here's what I'm going to propose:
We have our regular melee attacks, unchanged.
Then, we give them an AoE option with a relatively small radius centered on themselves, but that they can use every turn in place of their melee attacks, and this is meant to feel a bit like Destructive Wave. Because it's no recharge, we make it on par or slightly less damage when hitting two targets than focusing the longsword on a single foe - this is for when they are truly surrounded and can hit three or more targets. Also, I feel like it should be cold and necrotic rather than fire and necrotic, because I like associating cold magic with undeath (perhaps a nod to Warcraft's Scourge).
Then, we give them a really nasty recharge ability (our stunning dread wail). This might then take the place of a fear aura, as we're getting that effect from this. This is going to be psychic damage.
For legendary actions, it might be a little too powerful to let them make a full strike with just a single legendary action, but I think we'll allow it just because of how legendary this foe should be.
So, I think we give them a single longsword attack for 1 legendary action.
We then take two to give them their mobility thing, which disengages them, frees them of grapples and restraints, and moves them perhaps only 20 feet (rather than a full 30).
And then, I think we give them something a little different that costs 3 legendary actions.
Here's my thought: zombies and skeletons feel worthless against high level players because they are so likely to miss and deal pitiful damage. However, one spell fixes this: Danse Macabre. This adds your spellcasting ability to both their attack and damage rolls, which means your Zombie can be attacking with a +8 to hit and dealing 1d6+6 damage - which makes them an actual threat. (1d6+6 isn't that amazing, but when you have five of them, it adds up.) I think we thus give our DK an aura that buffs undead creatures other than the death knight. We're not really changing their stats, so the DK is only adding 4, but that's enough to make these minions more than a nuisance. So, I think the 3 legendary action thing will essentially be Danse Macabre, but we're going to go back and fold that buff into an aura the death knight projects.
Let's bang out this stat block! (Also, we're doing the full legendary treatment).
DEATH KNIGHT
Medium Undead, typically Chaotic Evil
Armor: 20 (plate, shield)
Hit Points: 180 (19d8+95)
Speed 30 ft.
STR 20 (+5) DEX 11 (+0) CON 20 (+5) INT 12 (+1) WIS 16 (+3) CHA 18 (+4)
Saving Throws: Dex +6, Wis +9, Cha +10
Damage Immunities: necrotic, poison
Condition Immunities: exhaustion, frightened, poison
Senses: darkvision 120 ft., passive Perception 13
Languages: Abyssal, Common (I might add "any languages they knew in life.")
Challenge 17
Aura of Unholy Command: Unless it is incapacitated, the death knight radiates an aura of unholy will, empowering nearby undead. Other undead creatures of the death knight's choice that are within 60 feet of the death knight add the death knight's charisma modifier (+4) to their attack and damage rolls. The death knight and creatures affected by this aura also have advantage on saving throws against effects that turn undead. A creature can only benefit from one of these auras at a time.
Legendary Resistance (3/day). If the death knight fails a saving throw, it can choose to succeed instead.
Magic Resistance. The death knight has advantage on saving throws against spells and other magical effects.
Actions:
Multiattack. The death knight makes three longsword attacks.
Longsword. Melee Weapon Attack. +11 to hit, reach 5 ft., one target. Hit: 9 (1d8+5) slashing damage, or 10 (1d10+5) slashing damage if used with both hands, plus 18 (4d8) necrotic damage.
Remorseless Chill. A cloud of darkness swirls around the death knight. Each creature of the death knight's choice within ten feet must make a DC 18 Constitution saving throw. On a failure, the creature takes 17 (5d6) cold damage and 17 (5d6) necrotic damage, and their speed is reduced by 10 feet until the start of the death knight's next turn. On a successful save, they take half this damage and their speed is unaffected.
Dreadful Wail. (Recharge 5-6): The death knight unleashes an ear-splitting howl, assaulting their foes with agonized terror. Each creature within 60 feet of the death knight that is not undead and can hear it must make a DC 18 Wisdom saving throw. On a failure, the creature takes 65 (10d12) psychic damage and is frightened of the death knight for 1 minute. If the creature fails this saving throw by 5 or more, the creature is stunned for 1 minute as well. A creature can repeat this saving throw at the end of each of their turns, ending both conditions on a success.
Reactions:
Parry. The death knight adds 6 to its AC against one melee attack that would hit it. To do so, the death knight must see the attacker and be wielding a melee weapon.
Legendary Actions:
The death knight can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. The death knight regains spent legendary actions at the start of its turn.
Longsword. The death knight makes a longsword attack.
Wraith Walk (Costs 2 actions). The death knight ends the grappled and restrained conditions on itself, if it is affected by them, and moves 20 feet in any direction. This movement does not provoke opportunity attacks.
Marshall Undead Forces (Costs 3 actions). The death knight magically conjures five undead creatures that each use the zombie or skeleton stat block. These creatures are friendly to the death knight and obey its commands. They share the death knight's initiative, but act immediately after its turn.