Searing Soul.

Your magic stems from an arcane fire within you that can't be quenched. This fire burns within you, sometimes with such an intensity that magic radiates from you like heat. Many tiefling sorcerers have this infernal spark thanks to their fiendish ancestry, whereas others forge such an arcane connection through contact with an elemental or phoenix.
Regardless of your origins, your sorcery is unwavering and intense. Fire magic, in particular, comes as naturally to you as breathing and can be easily made to burn hotter, faster, and more explosively.

Level 1 Features:

Flame Speaker.

The arcane magic you command is infused with elemental fire. You can speak, read, and write Primordial. Knowing this language allows you to understand and be understood by those who speak its dialects: Aquan, Auran, Ignan, and Terran.

Searing Spells.

Starting at 1st level, you learn additional spells when you reach certain levels in this class, as shown on the Searing Spells table. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know.

Ignition.

Starting at 1st level, the flames you create with magic are pulled from within you. Whenever you cast a spell that deals fire damage, your skin crackles with embers, and until the start of your next turn, your speed increases by 10 feet and opportunity attacks against you are made with disadvantage.

Level 6 Feature:

Burning Blood.

By 6th level, you gain resistance to fire damage, and whenever you deal fire damage to a creature, you can choose to ignore any resistance it has to fire damage.
In addition, you gain one of the following options (typically based on your sorcerous origin). Once you've used this option, it can't be used again until you finish a long rest unless you spend 3 sorcery points (no action required) to restore your use of it.
Phoenix Rise: When you're subjected to fire damage, you can use your reaction to take no fire damage and instead regain a number of hit points equal to half the fire damage dealt, before reducing it in any way. When this happens, you can immediately fly up to 30 feet, without provoking opportunity attacks.
Summon Hell Hound: You can use an action to summon a hell hound in an unoccupied space that you can see within 30 feet of you. The hound remains for 10 minutes, until you end the effect (no action required), or until it drops to 0 hit points. For the duration, the hound is friendly to you and your companions. In combat, it shares your initiative count, but it takes its turn immediately after yours. It obeys your verbal commands (no action required by you). If you don't issue any, it takes the Dodge action and uses its movement to avoid danger.
Volcanic Eruption: You can use an action to choose a point on the ground that you can see within 60 feet of you. Lava erupts from that point, causing the ground in a 20-foot radius to buckle upwards. Each creature within the area must succeed on a Dexterity saving throw against your spell save DC. On a failed save, a creature takes 3d6 bludgeoning damage and 3d6 fire damage and is knocked prone. On a successful save, a creature takes half as much damage and isn't knocked prone. The ground then returns to normal, unless you decide to concentrate on the effect (as if it were a spell), to a maximum of 1 minute. For the duration, the area is considered difficult terrain, and when a creature moves into or within the area, it takes 1 fire damage for every foot it travels, unless it has resistance or immunity to fire damage.
A creature within the area that doesn't move on its turn takes 10 fire damage instead.

Level 14 Feature:

Explosive Speed.

Starting at 14th level, whenever you gain the benefits of your Ignition feature, you can take the Dash or Dodge action using a bonus action. In addition, until the start of your next turn, any creature within 5 feet of you that hits you with a melee attack for the first time on a turn also takes fire damage equal to half your sorcerer level.

Level 18 Feature:

Heart of Fire.

By 18th level, you gain immunity to fire damage, and your chosen Burning Blood option improves in the following ways.
Phoenix Rise: You also gain a flying speed of 60 feet for 10 minutes.
Summon Hell Hound: The hound is summoned with a number of temporary hit points equal to your sorcerer level, and it remains for 8 hours, instead of 10 minutes. It uses your proficiency bonus instead of its own. In combat, the hound uses your Charisma modifier for its attack and damage rolls, instead of its Strength, and the save DC for its Fire Breath is equal to your spell save DC.
Volcanic Eruption: Both damage types increase by 1d6 (to a total of 4d6 bludgeoning damage and 4d6 fire damage), and you don't need to concentrate on the effect for it to remain for the full duration.