Pyromaniac

Origin of Fire

Whenever you start casting a 1st level or higher spell that deals fire damage, fire begins to cover you all over. This state lasts for 1 minute and every time you cast another spell that deals fire damage this clock restarts.

While in this state you gain these benefits;

-creatures that you hit with a spell (including cantrips) that deals fire damage also take fire damage equal to your proficiency modifier this goes up every level, for example if you have +3 proficiency, you add 3 to firebolt, 6 to burning hands, 9 to scorching ray and so on (stops adding up at 20),
-creatures within 5 feet of you take fire damage equals to half of your sorcerer level (minimum of 1) at the end of your turn.

Fire in the Veins

At 6th level, you gain resistance to fire damage. In addition, spells you cast ignore resistance to fire damage.

Heart of Fire

When you reach 6th level you are able to control your fiery state way better. While you are in the effect of "Origin of Fire" you may end the undying flame on your body with a blast.

As an action you can let the flame on you go. This causes an explosion in a 15-feet radius which deals 6d6 fire damage.

The damage becomes 8d6 at level 14 and 10d6 at level 18.

You can use this feature a number of times equal to your proficiency bonus.

Pyromancer's Fury

Starting at 14th level, when you are hit by a melee attack, you can use your reaction to deal fire damage to the attacker. The damage equals your sorcerer level, and ignores resistance to fire damage.

Fiery Soul

At 18th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.

If you will, you can ignore the immunity as a whole and deal full damage but in order to do that you have to spend 3 sorcery points. (You can decide after you roll the damage rolls but before the DM determines the outcome.)