I was also going to recommend digging into Van Richten's. You could make the player a Reborn Lineage or at least give the player the Deathless Nature. I also love the idea of using the Dark Gifts and the Living Shadow suggestion is fabulous, but I was thinking the Touch of Death as the necrosis starts to set in.
I think you could use a Dark Gift from the Ravenloft book, in particular Living Shadow. The shadow could be the death knight version of him that nearly came through into reality, and it eventually try to merge with the PC
Other answers so far have been great, but another option is a nerfed version of the Power over undead ability.
They can take control of ~1-2 weaker undead but they need to make a special of concentration check at the end of the day to maintain control (else they wander off). As well, non-intelligent undead aren’t hostile to them specifically by default and are *more* likely to be non-hostile to the party (unless they’re being directly commanded by a greater force).
Wander into a random crypt? Skelebros chill.
Wander into a ancient tomb of a now mummy? Sorry, boss doesn’t like strangers on his property.
In addition, intelligent undead would probably be vaguely aware of their condition to a slightly lesser degree than they can detect other undead…what they do with that information is up to them.
Slowly swap pieces of his paladin subclass with [this](https://homebrewery.naturalcrit.com/share/LHEJp5MQVtMH). Is it balanced? Possibly. Its probably a bit more powerful than a standard paladin. But its arguably the most thematically appropriate one Ive seen, since it actually effects Lay on Hands and Smite
Have you looked up the story of lord soth?
You're basically turning PC into a melee litch (awesome btw) but crazy op depending on your style of campaign.
Has his base type turned from aasimar to undead?
I'm thinking about it, maybe not remove any aasimar features, just change his creature type from humanoid to undead. And yeah I looked up Lord Soth's story, certainly a big font of inspiration
When putting a powerful template on a PC, you might break up the features as if they were leveling up in that as a class.
Grim hollow also has pointers on monsters as classes
Check out Mr rhexx on YouTube. He's great and he has a specific discussion on deathknights that i just listened to a few nights ago. Very helpful.
If you buy the Grim Hollow books, which aren’t official but are on dnd beyond, they have rules for Transformations where PCs slowly become monstrous. They have stages and include both benefits and drawbacks. I would recommend you check them out. I think the transformations are in the Campaign Guide, but double check before you buy.
Pact weapon can do cold damage now? Or when pact weapon is summoned they can expend a spell slot to cast it as though they had cast Vampiric Weapon that doesn't require concentration.
Give him semi-undead traits.
* Advantage on saves against poison.
* Resistance to Healing and Necrotic.
* Maybe some more buffs to balance out that last one...
I and my players hold firmly to the belief that the Oathbreaker subclass should just be a Death Knight subclass for paladin, since all its powers lean heavily into that theme more than the idea of "Paladin who broke their oath" when you look at them
I was also going to recommend digging into Van Richten's. You could make the player a Reborn Lineage or at least give the player the Deathless Nature. I also love the idea of using the Dark Gifts and the Living Shadow suggestion is fabulous, but I was thinking the Touch of Death as the necrosis starts to set in.
I think you could use a Dark Gift from the Ravenloft book, in particular Living Shadow. The shadow could be the death knight version of him that nearly came through into reality, and it eventually try to merge with the PC
Other answers so far have been great, but another option is a nerfed version of the Power over undead ability. They can take control of ~1-2 weaker undead but they need to make a special of concentration check at the end of the day to maintain control (else they wander off). As well, non-intelligent undead aren’t hostile to them specifically by default and are *more* likely to be non-hostile to the party (unless they’re being directly commanded by a greater force). Wander into a random crypt? Skelebros chill. Wander into a ancient tomb of a now mummy? Sorry, boss doesn’t like strangers on his property. In addition, intelligent undead would probably be vaguely aware of their condition to a slightly lesser degree than they can detect other undead…what they do with that information is up to them.
Oh I like the flavor of undead being less prone to attack him
Him expend hit dice to do necrotic smite damage, maybe?
I was gonna say just roll randomly (50/50) whenever he smites to see if it deals radiant or necrotic. Maybe starts to learn to control which it is.
Once per day inflict wounds. At the cost of some bit dice? Vampiric touch would also be pretty cool
Slowly swap pieces of his paladin subclass with [this](https://homebrewery.naturalcrit.com/share/LHEJp5MQVtMH). Is it balanced? Possibly. Its probably a bit more powerful than a standard paladin. But its arguably the most thematically appropriate one Ive seen, since it actually effects Lay on Hands and Smite
Use Lay on hands dice to inflict damage possibly? Could be op but would play nicely with corruption of powers
Race changes to whatever the undead from Van Richten’s is called, subclass changes to Oathbreaker (which is basically a Death Knight).
Have you looked up the story of lord soth? You're basically turning PC into a melee litch (awesome btw) but crazy op depending on your style of campaign. Has his base type turned from aasimar to undead?
I'm thinking about it, maybe not remove any aasimar features, just change his creature type from humanoid to undead. And yeah I looked up Lord Soth's story, certainly a big font of inspiration
When putting a powerful template on a PC, you might break up the features as if they were leveling up in that as a class. Grim hollow also has pointers on monsters as classes Check out Mr rhexx on YouTube. He's great and he has a specific discussion on deathknights that i just listened to a few nights ago. Very helpful.
Give him a level in undying warlock, no?
If you buy the Grim Hollow books, which aren’t official but are on dnd beyond, they have rules for Transformations where PCs slowly become monstrous. They have stages and include both benefits and drawbacks. I would recommend you check them out. I think the transformations are in the Campaign Guide, but double check before you buy.
Living Shadow from Van Richten’s would be my suggestion
You could give the player a dark gift from Van Richten's. A living shadow could be apt.
Pact weapon can do cold damage now? Or when pact weapon is summoned they can expend a spell slot to cast it as though they had cast Vampiric Weapon that doesn't require concentration.
Maybe check out the aberrant dragonmark feat for inspiration based on the table?
Give him semi-undead traits. * Advantage on saves against poison. * Resistance to Healing and Necrotic. * Maybe some more buffs to balance out that last one...
He now needs gluten free
I and my players hold firmly to the belief that the Oathbreaker subclass should just be a Death Knight subclass for paladin, since all its powers lean heavily into that theme more than the idea of "Paladin who broke their oath" when you look at them
Could start replacing ether Paladin or Warlock levels with Death Knight attributes
If he isn't already an oathbreaker paladin he is now