I'm a big fan of the ARPG Path of Exile (lot of fun and totally free to play if you wanna check it out)
In the game, there is a spell called Righteous Fire, that causes you to burn for a percentage of your health as fire damage and in turn it burns all foes around you for fire damage based on a percentage of your health as well.
I really like the flavor of the ability, shrouding yourself in fire and slowly burning to death while also burning your foes around you.
Anyone have any ideas on how to translate that theme into a custom spell for pathfinder?
Having a spell that deals hit point damage to the caster and the deal that damage to all foes in a set radius shouldn't be to hard to do. The difficulty is placing it so it won't be too strong or too weak for that level. Having self burn spells could be a speciality of some Servents of a fire God, or wild mystical types. At the moment, I think it could be a level 2-3 spell if it used a die of self damage, then the same die amount dealt to the enemy.
So a d4 self harm could hit a radius of 15 meters+caster level with d4 of damage. Very risk reward, as the player might need to balance the level of the die used(up to a d12) of self injury, with the level of life they have, as well as the damage they could deal. Might think that adding modifier to damage dealt to others(wis,Cha, or int) to limit the amount of 10 self-1 others there is. My guess is have the modifier being the spell casters spell casting trait.
Self burn spells could be insanely good in certain situations if the balance was done right. Some might drain con, to deal a massive damage attack to a group of foes, others reduce max hp to unleash a salvo of blasts that incinerate foes. And with the right gear and companions, you pay less for you powerful Attacks.
Blazing Aura (Combat)An inferno rages inside you, causing your body to radiate intense heat.
Prerequisites: Inner Flame, Scorching Weapons, character level 13th, ifrit.
Benefit: When you use Scorching Weapons, on your turn as a free action, you may create an aura of heat that lasts for 1 round. This aura deals 1d6 points of fire damage to any creature that begins its turn adjacent to you.
And there are a few more abilities that effectively do what you are asking (minus the damage to self) across the level ranges and classes. You might be trying to reinvent the wheel when it could be more effective go search the PRD or d20pfsrd for what fits your need.
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What spell level are you aiming for?
Spells that hurt the caster are somewhat tricky to balance, especially if the caster can render themselves immune to that sort of damage. Also, spells with damaging effects that emanate from the caster have limited worth for player characters because they have to be close to their opponents and will potentially harm their own allies.
With this sort of thing, I would say direct damage to the player, with no reduction(they are deliberately inflicting damage on themselves) to get the effect. Balancing it is tricky, but I would say limiting the damage amount you deal(+modifier) to the same die type as the one you self inflict. A d4 to self is a d4 to the enemies. And it is not a magic missile. It is not auto hit.
And as for when it is used, I would say it should be used when you are surrounded. Good choice for bloodragers and offensive frontline clerics, who are already close to the enemy and have better use for aura style blasts.
The name of the attack in the game was righteous fire, which feels like a lawful or good spell for clerics,oracles, Paladins or warpriests. Consuming blaze sounds like the kind of name this spell might have, since the fire burns everything, including the caster. Many Goblins sorcerers might go out this way, burning themselves to ashes along with their enemies and rivals.