The ability to change and modify a character or monster without adding class levels or applying templates is something that D&D just doesn’t really do; however, d20 Modern introduced a system of Mutations that do exactly that. I’ve brought that system over for Aldhaven while modifying and expanding it to work within the game a bit better.
Thematically, mutations are limited to one of two types: physical and psionic. Physical mutations cause physical changes, like adding fangs or claws, giving unnatural flexibility, or greatly strengthening the immune system. Psionic mutations can grant psi-like abilities or other supernatural abilities powered by psionic ability. Mutations never have an arcane, divine, or other magical power or ability.
Mutation Points
The “currency” used to gain mutations is called Mutation Points. Each mutation costs a number of MP (or grants MP if it’s a drawback), which a character must spend in order to gain that mutation. Mutation Points can be gained in several ways. They might be gained through exposure to harmful conditions, like radiation or taint, intentionally gained through things like the Beneficial Mutation feat or Mutated Creature template, or even simply by asking the GM if you can add mutations and drawbacks to your character.
A character can save Mutation Points and pool them to purchase a more expensive mutation later, but a character cannot ever have a negative number of MP.
Types of Mutations
Cosmetic
Cosmetic mutations are just that: cosmetic. They are a mechanical to way to say, “Hey, I have this special trait,” without granting any actual game benefit (beyond simply having a mutation, which lets you qualify for the Beneficial Mutation feat). Cosmetic mutations cost no MP to gain.
Drawbacks
These are negative mutations, those that only inflict penalties with no associated benefit. Their effects can range from reducing ability scores to making you susceptible to cold to causing your arms to wither away to having uncontrollable panic attacks whenever combat starts. Drawbacks grant a character additional MP that can be spent to gain beneficial mutations.
Mutations
Beneficial mutations are simply called “Mutations.” They come in two categories, which differ only in degree. Minor Mutations cost 3 or fewer MP, provide a minor benefit, and typically are not too obvious at a glance. Major Mutations cost 4 or more MP, grant a significant benefit, and often cause drastic and obvious physical changes.