A Heffith begins every run already changing, with one mutation taken hold, and grows another with every experience level. The new Mutations tab on the character sheet lists what has taken hold, and its tree shows what could come next.
Mutations grow in six lineages: Piscine, Lupine, Avian, Serpentine, Chiropteran and Saurian. A mutation appears only once the one it grows from is carried, so each lineage runs from a faint first change to an unmistakable one, and a lineage already started is three times as likely to deepen as a new one is to begin.
Every mutation gives and takes, and a first change costs more than it gives. One sharpens the senses, one hands over a perk without its requirements, one adds an Action Point to turns started in a flooded room, underground or under open sky, and some take one away in the wrong place. The deepest reshape the body entirely: a tail keeps leggings off, horns a helmet, wings body armour, talons boots. A changed body also costs Comfort every turn, more for every lineage mixed in, and every mutation others can see costs Charisma. No stat falls below 1.
A Heffith can fight back. Suppression spends experience to put down a mutation that has nothing grown out of it, and it never returns. The price doubles every time, and experience spent on your body is experience not spent reaching your next level.
Nobody else is safe either. Blight fruit carries a 1% chance of a mutation, and for someone already changed, one time in ten it takes a mutation away instead. The Stabilized Arcane Vial costs 10 Health to drink and carries a 10% chance. Any race can be changed, and the first mutation opens the tab.