Cinderkin are what happens when the two elemental lineages meet: Earthkin, humanoids whose bodies are made of flesh and earth, and Emberkin, humanoids whose bodies are made of flesh and fire. Stone that holds heat. Skin that cracks and shows what is burning underneath.
Under the Mixed Ancestry rules a Cinderkin takes one trait from the top of one lineage and one from the bottom of the other, which means no two need resemble each other much. The parent lineages give them this to choose from:
Earthkin
Stoneskin: Gain a permanent +1 bonus to your Armor Score and damage thresholds at character creation.
Immovable: While you're touching the ground, you can't be lifted or moved against your will.
Emberkin
Fireproof: You are immune to damage from magical or mundane flame.
Ignition: Mark a Stress to wreathe your primary weapon in flame until the end of the scene. While the weapon is ablaze, it gives off a bright light, and you gain a 1d6 bonus to damage rolls with that weapon.
The pairing decides what kind of Cinderkin you get. One who keeps the earth's Stoneskin and the ember's Ignition is armored and burning and not fireproof — able to set their own weapon alight, and able to be hurt by it.
What is actually known
One Cinderkin: Temperance Blacksand, who assembled on the black-sand beach at The Cinderline in the hour the first colossus came out of the ground, with fine grains of essentia worked through their body.
That is the whole record. Whether there are others — whether Cinderkin are a people, a thing that happens where fire and stone and a god's spilled power meet, or a thing that happened once — nobody in the Drylands can say.
Source · Hope & Fear · Daggerheart Core Rulebook