Volcanic Rage

Starting when you choose this path at 3rd Level, you can turn yourself burning hot when you rage. If you do so, for the duration of your rage you, your skin, and weapons shines like hot ember shedding 10 feet bright light and 20 feet of dim light around you. Any creature who touch you or hit you with a melee attack while you are in a volcanic rage take fire damage equal to half your level, rounded up. Additionally, all your attacks deal an additional 1d6 fire damage.

Once you've used this feature you can't use it again until you finish a short rest.

Incombustible Body

When you pick the Path of The Molten Core at 3d level your strong connection to the molten core of the earth and its many volcanoes makes you resistant to fire damage while raging.

Starting at 6th level, you are resistant even when you're not raging and have adapted to environments of extreme heat, as described in chapter 5 of the Dungeon Master’s Guide. Additionally, any gas or smoke caused by fire or magma has no poisonous effects on you.

Fire And Smoke

Starting at 6th level you learn two new attacks that you can use in the place of one attack action on your turn as described below. If the effect of the attack require a saving throw, the DC equals 8 + your proficiency bonus + your Constitution modifier. You can use these attacks a number of times equal to your proficiency bonus. You regain expended uses after you finish a short or long rest.

Burning Grip. Your hands glow burning hot and you make an unarmed melee attack against a creature within range. On a hit, the target takes 2d8 fire damage. Alternatively, you can choose to direct the attack to a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that the creature is wearing. On a hit, you cause the object to glow hot red and any creature in physical contact with the object takes 2d8 fire damage. If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn't drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.

Breath of Smoke. You breathe out a 20-foot-radius sphere of heavy dark smoke centered on you. The sphere spreads around corners, and its area is heavily obscured. It lasts for up to one minute or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it. Each creature other than you that is completely within the cloud at the start of its turn must make a Constitution saving throw. On a failed save, the creature spends its action that turn coughing and gasping for air. Creatures that don't need to breathe or are immune to poison automatically succeed on this saving throw.

Volcanic Eruption

Starting at 10th level, you can while in your volcanic rage, use an action cause a minor earthquake and eruption.

The earth begins to shake and tremble as a crack forms in a straight line from where you stand to the point you choose within 60 feet of you. Each creature standing within 5 feet of the crack must make a Dexterity saving throw or fall prone on a failed save.

Additionally, at the point you choose a 5 feet wide crater forms and starts to erupt hot lava and fire hitting everything within a 15 feet radius of it. Any creature standing in the area must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one. The magma spreads around corners. It ignites flammable objects and melts glass and metal objects in the area that aren't being worn or carried. The affected area stays burning hot for up to 10 minutes or until soaked with water or ice. A creature takes 1d6 fire damage when it enters the affected area for the first time on a turn or starts its turn there.

The DC equals 8 + your proficiency bonus + your Constitution modifier. Once you've used this feature you can't use it again until you finish a short or long rest.

One With The Core

Starting at 14th level you are entirely immune to fire damage when you're raging. Additionally, if you use your breath of smoke feature while you rage you have blindsight in the area it affects while you're inside of it.