Immolator

The Immolator has a special relationship with the element of fire. They are drawn to it, despite its danger, and it responds by granting the use of its magnificent power to the one who sees it as something worthy of respect and even, sometimes, worship. Flame Incarnate is not a deity in the sense of most gods, governing its sphere of reality and blessing mortals; nor is it an Eldritch entity of cosmic power, shaking the world with its heat. Rather, Flame is an aspect of the universe so important and beautifully terrifying that it has gained the power of belief which manifests in many ways throughout the world and throughout time. 

Perhaps your sorcerer was assumed dead, burned at the pyre but that they burned not and rose anew, awakened by the fire like a Phoenix. Maybe this Immolator lost someone or something important to fire, and in return the cosmic balance allowed them to be blessed by Flame. It's possible that your character has always been what some may call "a pyromaniac," except that Flame loved them as much as they did Flame. 

Regardless of how your sorcerer gained the power of the Immolator, Flame Incarnate demands one thing alone: it must be used. Flame must burn, must spread - a cauterizing heat or a purifying cooking fire is good, but it feels so much better to let the fire wash over your enemies on the battlefield, destroying and defiling. Your character may have a difficult or unique relationship with the Flame Incarnate, but generally the Flame wants the sorcerer to use its power as much as the spellcaster.

Add Fuel to the Flames

Fire is chaotic in nature. The more it is used and the more powerful it grows, the harder it is to contain. An Immolator is able to increase the amount of Flame they can channel in combat, giving them a greater likelihood of satisfying the burning desires of the Flame. However, when the fire burns bright, the fire burns hot. The more you channel this extra boost of Flame, the more likely you are to lose control of it in the future.

Starting at first level, whenever you cast a fire-based spell, you can choose to add up to +4 to the roll or save DC; however, your threshold for a critical failure increases by the same amount as the roll bonus or twice the amount of the save DC bonus. You can continually use this ability, but the penalties stack. Every time you successfully cast a fire spell without this ability, the critical failure threshold decreases by 2; if you roll a critical fail, it decreases by 4 (to a minimum of 1). Otherwise, the threshold resets on a long rest.

Fire Warms the Heart

The Flame loves to burn and destroy, but it also honors its Immolators when the dust settles and the scene is quiet. If you stick by Flame in the calm moments, the Flame will reward you, will lift you up that you may continue channeling it.

When you short rest by a campfire or similar source of fire, you can heal HP equal to two hit dice without spending dice.

Friendly Fire

The Flame desires combat, but it will not do for your allies to burn with your enemies; Flame Incarnate is chaotic, but not evil. For Immolators who have proven themselves to the Flame, it allows a measure of control to direct it around certain targets. When the fires do reach your enemies and take the light from their eyes, Immolator and Flame alike are empowered.

Starting at sixth level, anytime you cast a spell using a spell slot which deals fire damage and requires a saving throw, you can act as if you used the Careful Spell metamagic feature, even if you do not know that feature. You can use this feature a number of times equal to your Charisma modifier and regain the uses after a long rest. Additionally, when you make the killing blow on a creature using fire damage, you regain one sorcery point. 

Backfire

When an Immolator comes under fire, fire is on their side. The Flame will not allow itself to be used against its own chosen mortals without a cost. When an Immolator allows Flame to right the offense and the offender is executed, Flame rewards the Immolator even more than usual.

Starting at fourteenth level, when you take fire damage, you can use your reaction to cast the spell Hellish Rebuke, ignoring distance and cover, at first level without a spell slot or the material components; if you do this, the damage you took is halved. If this kills the targeted creature, you regain an additional sorcery point to that which is granted by Friendly Fire.

The Immolator State

You are the favored one of the Flame, the avatar of its will, the one who speaks for Flame Incarnate as none other. You have united such that you can become one with the fires, not to act for it but to act as it. 

Starting at eighteenth level, as a bonus action, you can assume the Immolator State. In this form, whenever you cast a spell that deals fire damage, you deal additional damage equal to your Sorcerer level. In addition, you gain immunity to fire damage and heal a number of health points equal to your Charisma modifier when you cast a spell that uses a spell slot and successfully deals fire damage.
This form lasts for one minute and cannot be used again until after a long rest.