One of the exciting things that come with new rules-expansion books like Xanathar's or Tasha's or with campaign setting books like Guildmaster's Guide to Ravnica or Van Richten's Guide to Ravenloft is that we get new subclasses (technically Eberron: Rising from the Last War did bring three new subclasses, though they were for a new class.)
Recently, Tasha's consolidated many of these by publishing the entire Artificer class and the various subclasses from Ravnica, Theros, and a couple from Sword Coast Adventurer's Guide (though not all - some of those were reprinted in Xanathar's, and some didn't really warrant reprinting) in a setting-agnostic book. (Sadly we didn't get Exandria/Wildemount's Chronurgist, Graviturgist, or Echo Knight, which are all cool).
But one thing that is easy to forget is that even when 5th Edition was just its core three books, there were subclasses that were not found in the Player's Handbook. These options, the Death Domain Cleric and the Oathbreaker Paladin, are designed for villainous NPCs.
Now, I don't know about other DMs, but I tend to prefer to use or make simpler stat blocks over making a full character sheet for NPCs, even villains. Player characters are complex because the assumption is that the player is focusing their entire attention on piloting that one character. DMs need to run a whole bunch of different monsters. I think that's why, in recent adventures and books, we've seen spellcaster monsters given innate spellcasting with a handful of spells instead of a huge array of spells with different spell slots. It's just a lot easier to run that way.
So, building out an entire PC-like NPC with these subclasses is maybe a little much, and so I don't really imagine most people are going to use these subclasses for their initial purpose.
But, they are also fully functional (well, we'll get to that) subclasses for existing classes. Why not just let players try them out. After all, a player character can be of an evil alignment. Why not let them be a paladin who has forsaken their oath or a cleric devoted to a god of death?
I also like to play with shades of grey - what if the paladin had previously had an Oath of Conquest? Might they have broken their oath not because of a fall to evil, but a desire to break with a corrupt order? I recognize that Oathbreakers are meant to be more the bad kind of oathbreaking, and that Oath of Redemption might be a better fit for such a character. Still, I kind of like to play in an environment in which necromancy and such is not inherently evil - it's just how you use it.
So, if we want to make these subclasses playable, let's actually evaluate how they stack up.
Death Domain Clerics:
First off, domain spells. Death domain gets false life, ray of sickness, blindness/deafness, ray of enfeeblement, animate dead, vampiric touch, blight, death ward, antiflife shell, and cloudkill. I don't know that I'd say any of these is game-breakingly amazing. Clerics already have a fantastic spell list. Animate dead is certainly flavorful, and blight is a nice high-damage spell.
Next, death clerics get proficiency in martial weapons. Notably, they don't also get heavy armor, so you might be better served going with a dex-based weapon and not worry too much about getting high strength.
Then, we get the big level 1 bonus: Reaper. You learn one additional cantrip from the necromancy school (Toll the Dead's probably a good option) and if you cast a necromancy cantrip that targets only one creature, you can have it target two creatures if they're within 5 feet of each other. Consider also that Spare the Dying is a necromancy cantrip, and you can use this to stabilize two allies who are unconscious at a time (if they're next to one another.)
Level 2, death clerics get their special channel divinity option, which is Touch of Death. When you hit a creature with a melee attack (weapon or spell) you can use this to deal necrotic damage equal to 5 + twice your Cleric level (so 7 at level 2, and 45 at level 20.) This is a pretty huge boost to damage.
At level 6, your necrotic damage (which is kind of a theme here) from spells and the aforementioned channel divinity ignores resistance. Necrotic resistance isn't super common, but this is certainly not a bad thing to have, if a bit limited in scope.
At level 8, your divine strike bonus deals necrotic damage, unsurprisingly. Weirdly, the previous feature doesn't seem to cause this damage to ignore resistance. As usual, you get 1d8 extra damage per turn, and it upgrades to 2d8 at level 14.
Finally, at level 17, you get improved Reaper, which works the same but now counts for necromancy spells of 1-5th level as well. It will still consume material components for each target, but it's just one spell slot and one casting time. Note here that resurrection magic tends to be necromancy - meaning if two of your allies go down in a fight and they're next to one another, you could get both back up in a single turn. Likewise, you could blight two enemies standing next to one another.
The Death Cleric certainly feels focused on damage-dealing. I think that the Inescapable Destruction (ignoring resistance) feature is a little light, but Reaper and Improved Reaper have a ton of potency.
Now, let's look at the Oathbreaker.
At level 3, we'll start with oath spells. Oathbreakers get hellish rebuke, inflict wounds, crown of madness, darkness, animate dead, bestow curse, blight, confusion, contagion, and dominate person. I think Hellish Rebuke here is pretty cool. Inflict wounds is a bit odd because, as a paladin, you'd probably rather just hit them with a weapon instead of doing a melee spell attack.
You also get your two channel divinity options. The first is Control Undead. You can force an undead creature with a CR lower than your level to make a wisdom save, and if they fail, they have to follow your commands for 24 hours (or until you use this again.) This is actually pretty freaking cool - it's a concentration-free dominate monster (that admittedly only works on lower-CR undead.) Enlisting a vampire spawn to fight for you is pretty good. But even better is that you could send one to spy on their master for you. Hell, you could have one open up all the curtains in the castle right before sunrise!
The other Channel Divinity option is Dreadful Aspect. This causes all creatures within 30 feet of you that you choose to have to make a Wisdom save or be frightened for 1 minute, and they only get to repeat the save once they get more than 30 feet away from you. This is a pretty powerful effect as well.
At level 7, you get Aura of Hate. And this one gets a little tricky. You and any fiends or undead within 10 feet (30 feet at level 18) gain a bonus to melee weapon damage rolls equal to your Charisma modifier (minimum of 1.) (And the bonus doesn't overlap with other Oathbreakers.) On one hand, this is actually huge - if my Vengeance Paladin had this, she'd get a total bonus of +10 to every hit between strength, her +2 weapon, and her +3 charisma. However, it says that all fiends or undead get this - meaning that if you're fighting fiends or undead, you're actually enhancing their ability to hurt you. I think if this were designed for players, there'd be something like an "allied fiends and undead" qualifier. And, certainly, if you use Animate Dead or that Control Undead ability, this will help it. On its own, though, it's a fantastic bonus to the paladin him or herself.
At level 15, they get Supernatural Resistance, which gives you resistance to nonmagical bludgeoning, piercing, and slashing damage. There are actually many monsters that do not have magic weapons, so this is a pretty huge bonus.
Finally, at level 20, you get Dread Lord. Once per long rest as an action, you can power yourself up for 1 minute with an aura of gloom. Any bright light within 30 feet becomes dim light. And if an enemy that is frightened by the paladin starts its turn in the aura, its takes 4d10 psychic damage (remember Dreadful Aspect?) In addition, the paladin and any creatures they choose are in darker shadow, so creatures who rely on sight get disadvantage on attacks against them. But wait, that's not all: on your turn while this is up, you can use a bonus action to attack a creature with the shadows. You make a melee spell attack against the target (if it's in the aura,) dealing 3d10 + your charisma modifier necrotic damage to them.
So, actually, yeah, the Oathbreaker has some really powerful features. Aura of Hate is amazing as long as you're not fighting fiends or undead, where it can potentially become more of a liability.
One thing I'll note is that when it comes to Clerics and Paladins, these are two classes that are very strong in general, relying less on subclasses for their power (I think the Ranger is the exact opposite - baseline they're kind of blah but then you get things like Gloomstalkers and suddenly you can see the appeal.) For what they bring to the class, I think the Oathbreaker is the more dramatically cool one. Death clerics primarily benefit from the power of Reaper and Improved Reaper, but you're a cleric, so you'll be fine.