Hellfire sorcerer

 Soul of Fire

At 1st level you are immune to the damage of any of your own spells that are affected by the Flames of Hell metamagic ability.

Flames of Hell

At 1st level you gain access to a metamagic ability that allows you to alter the elemental composition of your spells. By expending Sorcery Points equal to the spell’s level, you may change the damage type of any spell that deals fire, cold, lightning, acid or poison into hellfire, which deals half fire, half necrotic damage (round fractions down). This metamagic ability may be used in conjunction with another metamagic ability, assuming you have adequate sorcery points available.

Clinging Flames

At 6th level, whenever you deal damage to a target with a spell of first level or higher that deals fire and/or necrotic damage, the creature is affected by clinging flames and continues to burn, taking an additional amount of that damage type equal to ½ your level at the start of its next turn.

Hellwalk

At 14th level you can teleport to a location you can see within 100 feet. At both your origin and destination points there is a burst of hellfire that deals fire and necrotic damage equal to your level (half of each type) to all adjacent creatures. You regain the use of this ability after a short rest.

Core of Hellfire

At 18th level you become immune to fire and necrotic damage. Additionally your Clinging Flames ability now lasts for 2 turns rather than one

Soul of Fire

 At 1st level you are immune to the damage of any of your own spells that are affected by the Flames of Hell metamagic ability.

Flames of Hell

By expending Sorcery Points equal to the spell’s level, you may change the damage type of any spell that deals fire, cold, lightning, acid or poison into hellfire, which deals half fire, half necrotic damage (round fractions down). This metamagic ability may be used in conjunction with another metamagic ability, assuming you have adequate sorcery points available. 

Clinging Flames

At 6th level, whenever you deal damage to a target with a
spell of first level or higher that deals fire and/or necrotic
damage, the creature is affected by clinging flames and
continues to burn, taking an additional amount of that
damage type equal to ½ your level at the start of its next
turn.

Hellwalk

At 14th level you can teleport to a location you can see
within 100 feet. At both your origin and destination
points there is a burst of hellfire that deals fire and
necrotic damage equal to your level (half of each type) to
all adjacent creatures. You regain the use of this ability
after a short rest.

Core of Hellfire

At 18th level you become immune to fire and necrotic damage. Additionally your Clinging Flames ability now lasts for 2 turns rather than one.