Fire is almost certainly the first element ever ensorcelled by man, or even the creatures before men. Unlike spirits, powers beyond, Gods, and even the other elements; fire is easy to work with. It is implicitly at rapt attention at all times; no complex ritual or long winded incantation or noise is needed to draw forth the spirit within. Fire only need be fed something strange to burn, or many things to burn, for an air of the mystic to surround it. It is easy to feed, sacrifices must be burnt in the fire and then are gone. You need not protect the ritual site for years to come.
Of course, Fire isn't perfect for sorcery. Its magic only lasts as long as the fire does. Spells of protection woven in the firelight fade with the fire's fading. Fire will in one minute maim your foe, and then the very night night the fire warms their home and cooks their food. It has no master. Embers only contain a fraction of its magic, and ash from a fire only contain a fraction of the fraction of the ember- you could burn down a city in a blaze and a mountain of ash would only contain the magic for a single cantrip. Even wind has more longevity then fire. The spell will come and go as its pleases, sure, but it will one day return. Fire will not.
But not all magic wishes to be short lived. When you sacrifice to the elements of stone and the mountain on a rock altar, the blood seeps into the earth and will not only serve as a power source for the sorcerer, but for the generation after his, and the next, and then the invaders long after who find the altar and shun it for its unknown magic. These things have memory. Fire does not. How do you then get something permanent, lasting, out of the magic of fire?
“Billow flame and smoking ash,
fevered flesh and bite of asp.
Choking fire, offerings feed,
Glowing ember, I call for thee.
Rising blaze, swallow stars,
Lasting heat within my scars.
Rise forth from the offering bowl,
Come, embrace me, I am Kindlesoul!”
Kindlesoul is a powerful magical ritual. It can only be cast by a powerful sorcerer, or in your game, probably using a 5th level spell slot or greater. In order to cast the ritual, the user must sacrifice many objects or beings to the elemental fire in order to gain its favor, and then is consumed by the flames.
The Kindlesoul is a person who has endured the ritual. All over their body, spiral flame scars are burned into their flesh. These scars have lasting heat, warm to the touch even years after the ritual, but more skilled practitioners have these scars tattoo their skin gracefully, reducing or eliminating the permanent pain these cause the user. The ritual deals 2d6 damage and reduced their maximum hit points by the same amount, which is a permanent reduction. You can also choose to “drink” the fire instead to burn the inside, but this deals 3d6 damage and hit point reduction, plus an additional -1d6 to your constitution score, all permanent loses, but this hides the evidence of this magic on your body.
The Kindlesoul gains resistance and takes half damage from all fire based attacks or spells. They can hold or touch standard campfire sized flames or smaller without injury. Candlewax slides off their skin painlessly. Secondly, the Kindlesoul may light things on fire at their touch; usually something dry or something that would light on fire if a torch was held up to it for a few seconds. The Kindlesoul can use this power through any part of their body, and may make flames kick up around their feet or may light the ends of branches to act as a torch as they touch it with their fingers, and so on.
Finally, the Kindlesoul gains a permanent magical boon with the elemental fire. They treat their level as one higher when commanding the spirit of fire or casting a fire based spell. This magical energy could instead apply to all spells; increasing in mystic power permanently.