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.
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.
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