How might this homebrew undead hybrid character type unbalance combat compared to non-undead creatures?

Question

About a week ago in-game (half a year IRL), a series of deadly events occurred resulting in two PCs becoming undead. One used to be a tabaxi, the other a high elf. Both of them are now undead versions of themselves: the tabaxi by making a pact with a neutral evil demi-god of the Death Plane, and the other by dying and being resurrected as a result of this pact-making process.

According to What are the mechanical consequences that arise for a PC with the undead creature type?

By RAW, per Quadratic Wizard's answer:

Undead type has no inherent mechanical effect in D&D 5e.

So ever since these events happened, I've been finetuning the following homebrew traits.

Characters that became undead

• Hybrid Nature.[*] You have two creature types: humanoid and undead. You can be affected by a game effect if it works on either of your creature types. For the humanoid traits, use your original race description. Additionally, you gain the following traits from being an undead humanoid:

• Undead Defenses. Vulnerable to radiant damage; Resistant to necrotic damage.

• Undying Resilience. Cannot be healed by conventional healing (e.g. spells, potions), but can sometimes be healed by cantrips from the necromancy school (such as Chill Touch). Make a Constitution save (with advantage) versus the spellcaster's Spell Save DC. On a success, you are healed for the amount of damage dealt. On a fail, nothing happens.

• Desecration. Advantage on ability checks, saves and attack rolls while standing on a desecrated surface.

• Ill Will. 14th level necromancers can command you at will, while you automatically fail the first three Wisdom saves.

Question

I'm trying to find a balance between benefits and downsides for becoming undead, so that undead humanoids are more powerful in combat than mere mortals, but not so strong (or without significant handicaps) that everyone wants to turn undead. My reasoning for this: with great risks come great rewards, but even greater downfalls. With "mere mortals" I mean (player) characters that are "just" humanoid while of the same level and class. So preferably the undead traits carry a danger to them, to keep the more careful characters (and players) from wanting to delve into such practice. Through interactions with the world setting and narration the dangers of undead seem clear to the players, but I'd like to emphasise this further through combat mechanics (of which some they can still discover).

The party has found out about all undead traits mentioned above, except from the final two (Desecration and Ill Will). I'm wondering whether I should add an additional benefit (such as a raised Constitution, or some other defense), and whether the desecrated grounds feature is too powerful already for a PC to have (as soon as they find out). I'm mostly wondering how the current undead hybrid type will unbalance combat at my table.

Other possibly relevant details regarding my campaign:

• Party composition (level 12): minotaur paladin of conquest, animated armor eldritch knight/wizard, undead tabaxi ranger/assassin/warlock, undead high elf mastermind, tiefling warlock.
• Every PC has Healing Surges.
• The main story arc heavily features undead creatures: mostly enemies but some (powerful) allies too.
• The BBEG is a devil attempting to become a lich (so will also become undead as a result).
  • From party level 15 onwards, an apocalyptic inter-planar war strikes down on the Material Plane (featuring mortals, angles, demi-gods, devils, necromancers and countless undead). The party is currently preparing for this coming war.
  • Current allies of the party include an important figure within the clergy of Helm (one of the most popular gods in my world, though only this one cleric knows of their undead nature), some secret wizards, and the mentioned demi-god (undead necromancer) of the Death Plane.
  • The organisation that hunts down the party consists of radical extremist paladins of Tempus that bring "redemption" to all magic users – they plan to slay 'm all. These are adversaries by default, but some individuals could change their minds by the party's initiative.

_{[*] based on Unearthed Arcana: Centaurs and Minotaurs}

Answer 1

Let's take a look at each of the features of this "Racial Template" (a term that hasn't really been used since 3.5, but I think is appropriate for what you're doing with this) and see how they stack up:

Hybrid Nature

In most situations, this feature is strictly downside. It expands the range of effects that can target you, and the vast majority of effects that can affect undead but not humanoids are negative for the recipient. For example:

• You receive extra damage from a Paladin's Divine Smite feature (notwithstanding the Undead Defenses feature you've also given them)
• You can be affected by spells like Protection from Good and Evil or Magic Circle

But also, because you're a humanoid

• You can still be targeted by low level charm spells like Hold Person, Charm Person, etc., despite being undead

How much this feature is downside depends on whether you regularly fight foes that could take advantage of these issues or not; my general sense is that most campaigns don't involve enemies that depend on spells like those to defend against PCs, so for now, we'll treat this feature as a "Minor Negative".

Undead Defenses

In most campaigns, Necrotic damage is more common than Radiant Damage. We'll call this a "Minor-Moderate positive".

Desecration

This feature is powerful, but desecrated land is pretty rare. It could be more common because of your campaign setting—stopping a Devil from becoming a Lich sounds like something that could involve a lot of Desecration, so to speak—but as DM you have control over that; therefore, this is "Minor positive"

Ill Will

This feature can be crippling, but it depends on how many 14th level Necromancers your party faces. Again: your setting seems like it could have quite a few, but you control that as DM. "Minor negative".

Undying Resilience

I saved this feature for last because it's the one that obviously needs tuning; not entirely for balance reasons, but that's definitely a part of it.

For starters, the ability to provide healing to a creature through use of a Cantrip must not be underestimated. In theory, this means that these characters can be fully healed between any combat encounters (not just one, like a Healing Surge/Short Rest might provide), so long as they have a spellcaster with access to cantrips that deal Necrotic damage; this is much more powerful a positive than the negative of being unaffected by "normal" healing spells is.

The fact that they require additional Saving Throws (more on that in a moment) only has combat implications, making healing-in-combat difficult. That's not nothing, but if characters can heal to full between combat encounters, it can make tuning combat more difficult, especially since both characters (theoretically) have ways to mitigate in-combat damage, depending on how many levels of Assassin the Tabaxi took (they need 5 levels for the Uncanny Dodge Rogue feature).

Sidebar: since Healing Surges feed off the Hit Dice pool a character would otherwise be spending in a short rest, this mitigates this issue somewhat; but Healing Surges are limited to once-per short rest, so it's not a total mitigation, and it still means that pool of dice becomes difficult to exhaust.

The second issue is that the feature is kind of kludgy, in two ways.

The first kludgy aspect is the additional saving throw to get healed, meaning every single cantrip cast in an attempt to heal one of these characters involves 3-4 d20s being rolled (1-2 for the Attack Roll or Saving Throw of the regular spell, and 2 for the Advantage roll of their Saving Throw), in addition to the actual damage/healing dice of the cantrip itself. That's a lot of dice, especially on the d20 side of things. I'm not necessarily saying the feature is unbalanced for this reason, just that it slows the feature down, is all.

The second issue is that how the feature calls out which healing is valid and which isn't needs to be better clarified: "conventional healing" is a very squishy term that could be interpreted in a lot of different ways, and if that's the wording you're going to present to your players, there's definitely going to be questions raised about what counts and what doesn't—stipulating "Potions and spells" won't cut it—especially because, lore-wise, those are the least conventional of any healing methods in Faerun.

Here's a few things to consider, though: the vast majority of spells that confer some degree of healing expressly call out Undead as invalid targets:

A creature you touch regains a number of hit points equal to 1d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs.

—Cure Wounds, Player's Handbook, pg. 230

So because of the Hybrid Nature feature these characters have, any healing spell that has this stipulation would not work on them. Even for spells that don't have this limitation, many of them are spells like Life Transference or Enervation, where the healing is provided by dealing Necrotic damage to another creature; you can make a call on this one, but thematically, I don't think it's inappropriate to allow these kinds of spells to restore health to these characters.

So the few left behind are spells like Aura of Vitality or Regenerate, the former of which is a class-exclusive spell (Paladin) and the latter of which is a 7th level spell (and it doesn't do a ton of healing for its level; its benefits are mostly in the ability to regrow limbs & counteract Death Saving Throws). Then you have spells like Aid and Heroes' Feast, which can't stack (because they have durations) meaning you couldn't use them more than once in the 8/24 hour duration of each respective spell.

So because your other feature already handles this, I think it's appropriate to instead phrase this ability like this:

You cannot recover hit points from potions.

If you want to make sure that no spells may recover hit points (including the exceptions I listed above), you can revert to the previous wording, minus the "conventional healing" part:

You cannot recover hit points from spells or potions.

If you want to go with my suggestion, but make it explicit where you can or cannot recover hitpoints with spells:

You cannot recover hit points from potions, or from Evocation spells

_{protip: all the spells that exclude Undead come from the Evocation school; the rest are all Transmutation, Necromancy, or Aura of Vitality as the one odd spell out that is both Evocation and can affect Undead. There's also Healing Spirit as a Conjuration spell, but it does have this restriction.}

From this point, if you really want to allow Necrotic-damage Cantrips to confer healing, well, that's your call. I'd advise against it, but it's your campaign. I would recommend finding a different mechanic though: maybe get rid of the extra Saving throw and instead cut the damage in half (take into consideration the fact that the character has resistance to Necrotic damage anyways?).

---

Conclusion

So realistically, the one feature you need to cross-examine is Undying Resilience. All the rest are perfectly fine, ranging from "mundane/unimportant" to "powerful, but situational", and since there's a blend of both upsides and downsides, I don't feel the need to advise balance changes on those other features.

Answer 2

As a sidenote, you should look into the Revenant from Unearthed Arcana: Gothic Heroes. It's a well designed take on the same basic idea.

The Undead type is extremely significant

It's true that the Undead type comes with no inherent mechanical effect, but many mechanical effects interract with the Undead type. In that same answer, Quadratic Wizard lists several dozen such mechanical effects:

The drawbacks of being undead include:

• Cure Wounds, Heal, Healing Word, Mass Cure Wounds, Mass Heal, Mass Healing Word, Power Word Heal, Prayer of Healing, Spare the Dying do not affect you.
• You are subject to a cleric's turn undead, or certain paladins with the same ability
• A ranger with undead as a favored enemy is effective against you
• You are vulnerable to a paladin's Divine Sense and Divine Smite
• You can't benefit from a Life domain cleric's Channel Divinity
• You have disadvantage on an Oath of Vengeance paladin's Abjurt Enemy effect
• You can't benefit from a paladin's Lay on Hands
• 20th level paladins with the Oath of Devotion have advantage on spells cast by you
• 14th level necromancers can command you
• You cannot benefit from antitoxin
• Holy water deals damage to you
• Chill Touch, Sunbeam and Sunburst are more harmful to you
• Commune with Nature and Detect Evil and Good can sense you
• Dispel Evil and Good, Holy Aura and Protection from Evil and Good can affect you
• Forbiddance, Hallow and Magic Circle can exclude you
• You may be vulnerable to an Oathbreaker paladin's Control Undead
• You are vulnerable to the effects of the Helm of Brilliance, Holy Avenger, Mace of Disruption, a Scroll of Protection against undead, Sun Blade, Moonblade, Book of Exalted Deeds, Sword of Kas,
• You can trigger an enemy's Scarab of Protection
• You are vulnerable to a sprite's Heart Sight ability

Some of the advantages include:

• You are immune to the effects of the spells Antilife Shell, Blight, Command, Hold Monster, Phantasmal Force, and Sleep
• A necromancer can't benefit from Grim Harvest for killing you
• You can benefit from an Oathbreaker paladin's Aura of Hate
• You have an advantage when standing on desecrated ground
• You are immune to some of the special effects of the Nine Lives Stealer sword, Sword of Life Stealin and Blackrazor (which now actually heals you if you're hit with it)
• You are immune to various monster attacks, including the Banshee's Horrifying Visage and Wail, the Beholder's Sleep Ray, the Bearded Devil's Glaive effect, the Horned Devil's Tail effect, a Ghost's Horrifying Visage, a Ghast or Ghoul's Claws, a Jackalwere's Sleep Gaze, a Mummy Lord's Blasphemous Word, a Myconid's Rapport Spores, to name a few.
• You benefit from a Death Knight's Marshal Undead ability and a Mummy Lord's lair abilities.

Hence changing the creature type from Humanoid to Undead is an extremely significant change! So much so that a handful of new traits is comparativelly insignificant.

Trait analysis

The common theme is that alot of these traits are either redundant with the Undead type (in which case they are irrelevant to balance) or they inconsistent with existing 5e design (in which case they affect balance in weird or very situational ways).

Hybrid Nature

Undead creatures do not retain their previous type. Moreover, every creature in 5e has only one type, hence I strongly suggest removing the Humanoid type entirely, like the Centaur playable race from Guildmaster's Guide to Ravnica which is Fey instead of Humanoid.

Undead Defences

Usually only incorporeal undead (as in ghosts) are resistent to necrotic and even fewer are vulnerable to radiant. I presume the player characters still have bodies (like zombies or skeleton) and if that is the case this trait is entirely out of place.

Undying Resilience

Being incapable of receiving healing from spells is redundant with the Undead type. The ideas that potions do work on Undead and that necromancy spells can heal Undead is inconsistent with 5e Undead. Moreover, no cantrip should ever be capable of healing, because healing in 5e is meant to be limited.
Finally, it's extremely weird to use chill touch to heal because it normally prevents healing and gives disadvantage to attack by Undead.

Desecration

This traits works backwards compared to normal 5e design: this should't be a character trait, instead it should be a feature of the desecrated place itself.

Ill Will

This trait is unnecessarily redundant with the existing level 14 trait of necromancy wizards. It is also designed backwards, like the Desecration trait.

Alternatives

If you want your players to feel like undead in combat, then my suggestion is to look no further than the most common undead traits among corporeal Undead, namely Undead Fortituted and immunity to poison.

Getting traits that actually belong to common undead, immediatelly grounds the players in the fact that they have become like the many undead which they previously destroyed without second thought.

The social aspect

I would argue that the greatest drawback to being an undead (other than the fact that most undead lose either their mind, their body, or both) is the fact that they are incapable of integrating into society: zombies, skeleton, ghosts, and even liches are litterally horrifying, while vampires can't exist in sunlight and have to prey on their once-fellow humanoids.

On a fundamental level, Undead characters have to pretend to be alive or else they will be shunned and attacked by 99% of the people they meet. It's not merely discrimination, Undead characters are a mockery of life and the NPCs should behave accordingly. This is what you should focus on to truly portray the greater downfalls of Undeath.