How to adjust PC statistics when undergoing a (semi)permanent transformation into a different creature?

Question

The Monster Manual contains guidance for Player Characters as Lycanthropes (p. 207) or Vampires (p. 295).

Is there general guidance on how to handle player characters (semi)permanently transforming into another creature where they could retain their class levels and statistics but also take on characteristics of that creature?

Possible examples I am thinking of might be a transformation into a lich, a dragon, a fiend/celestial, etc.

I am specifically looking for solutions where the character can still function as a PC, gaining EXP, levels, and interacting with other player mechanics.

I am looking for answers based on official content, however, if there is no useful rules/guidance in DnD 5e, I would be interested in how other editions handled this idea or even DM/player table experience and solutions.

Answer 1

The answer is: There are no (official) rules on playing "Monster races"

Some humanoid "monsters" have been presented as playable races in supplements, such as orcs in Volo's guide (and Eberron: Rising from the war).

And older editions had supplements that allowed this, such as Savage Species for 3.5 edition, where monster abilities got divided up among levels so a PC would start as a much weaker version of a monster and gradually gain more of the typical monster abilities as they levelled up.

Starting off as a typical race and transforming into a monster would need similar rules or guides if you wanted the player to actually play, permanently, as such a creature, gaining XP, levelling up etc.

You can look at the druid's Wildshape as a comparison, where you take on a beast's form but maintain your mental stats and class abilities. But this is much more limited in creature type (beasts only) and the CR of the creature you can transform in to, in order to maintain balance.

This just gives an indication of how much more powerful it would be if a PC transformed into a lich or dragon.

So, no, in 5th edition there are no general rules or guidelines on this, and speculating/home-brewing on how you could do this would be outside the bounds of this forum.

Answer 2

Use Existing Races.

The simplest and most straight forward method is to replace the character's race with one which most closely aligns with the new monster. A celestial would be similar to an aasimar, a fiend similar to a tiefling, and a dragon similar to a dragonborn. This will create a subtle difference from their previous race and may not be as evocative of the change you wanted, but it will be the most balanced option by far. (As you are just using existing player options)

Consider Monster Templates.

Although not specifically designed for PCs, if you look at the template for half-dragon in the monster manual, you could apply a similar function to a PC without making the balance ramification untenable.

Even without using the Monster Template provided, it gives a framework for how you could draw some abilities from a monster and apply it to an NPC (or PC).

Level Adjustment Character Option From Previous Editions.

One way previous editions handled powerful racial choices is called Level adjustment. The level adjustment effectively replaces your first few levels. So for example, if you were playing a Black Dragon Wyrmling, you might effectively start at Level 2 or 5 (depending on if you wanted to consider CR or hit dice. And when you got enough experience to get to level 3 or 6 (again depending on the method) you would then be eligible to gain 1 level of whatever class you wanted. 5th edition is NOT designed around this! But if you were dead set on making it happen then it would at least be an entry point you could work from. From my experience you are basically playing the monster until you gain several levels.