Homebrew Infernal Dragon Species Details

A fire dragon egg that was stolen from it's nest and brought to the Nine Hells. It's been corrupted by Fiendish blood and magic and has turned this dragon into a hybrid of fiend and dragon.

Dragons of this caliber start out as medium and grow until they are the size of Ancient Dragons. As their power grows, they physically grow. These dragons have been corrupted by the energies of the Nine Hells but have only been influenced by it, and not under any Devil's control. They have the ability to take on a Tiefling's form but with Draconic features such as wings on their backs and scales around their bodies. 

A Infernal Dragon has the ability to use the armor they were in Tiefling form, absorbing the armor and enchantments into their scales as long as they have it on. They can use choose which AC to use while in dragon form, be it their natural Unarmored Defense or the armor they wear along with other abilities, including the ability to breath a combo of Necrotic and Elemental energy from their mouths in their original Dragon forms.

Death to an Infernal Dragon does not mean the end. They come back after 4d4 days (in game time. So hope the DM/GM lets a time skip happen or someone uses the Wish Spell or a Resurrection type spell to bring you back), their bodies being reduced to ash and any gear that is carried is dropped but any gear worn turns to ash as well, going with them when they come back to the Nine Hells. (Simply put: Weapons in hands are dropped while worn/unequipped weapons and armor/clothes are brought along unless a boon or ability states that an item has become a part of the Infernal Dragon such as the "Boon of Artifact Affinity").

(Gonna put a "Thank you!" out to Wingless62 for the idea and I have a few characters made off of their homebrew because I found it to be awesome. This is based on his/her homebrew. I am gonna publish this so my D&D group can find and see this but I don't mind sharing this for other's to use. You can find the homebrew this is based off of at: dndbeyond.com/races/527097-dragon )

 Details:

Bolt Holes

These dragons know of ways to get to the Material Plane from their home Level in the Nine Hells but have no way of returning back through the way they travelled. (Can only transport themselves).

 Breath Weapon

These dragons have the ability to breath fire and necrotic energy. After a long rest, they choose to either use fire or necrotic energy in their breath weapon attack. 

Ability Score Increase
Your Strength score increases by 2, your Intelligence sore increase of 1

Age
These dragons grow much quicker than normal, they reach maturity around 75 years. They are born with knowledge of what they are and what they were supposed to be but they believe they became a better race because of what they have become. 

Alignment

Infernal Dragons tend to lean towards Lawful Evil, but some may be extreme and turn into any alignment.

Size

Dragons are much taller and heavier than humans, as an adult standing 21 feet to 35 feet tall and weighing from 3.5 to 10 tons, or 7000 and 20000 pounds. Your starting size is Medium, and increases to Large at 5th level, Huge at 12th level, and Gargantuan at 20th level.

Speed
Your base walking speed in your original form is 30 feet, 30 feet climb, and 60 feet flying. This increases to 40 feet walking speed, 40 feet climb, and 80 feet fly at 5th level. In your Tiefling form, it's 30 feet walk and 30 feet fly at 1st level and flight increases to 60 feet at 12th level.

Languages

You know Draconic and Infernal.

Darkvision

You have darkvision for 60 feet and it increases to 120 feet at 5th level.

Blindsight

You have blindsight for 10 feet and it increases to 30 feet at 5th level and 60 feet at 12th level.

Infernal Dragon Traits

These are the traits of an Infernal Dragon

Dragon Breath

You can use your action to exhale destructive energy. Your draconic lineage determines the size, shape, and damage type of the exhalation. When you use your breath weapon, each creature in the area of the exhalation must make a saving throw, the type of which is determined by your draconic lineage. The DC for this saving throw equals 8 + your Constitution modifier + your proficiency bonus. A creature takes 6d6 damage on a failed save, and half as much damage on a successful one. The damage increases to 10d6 at 5th level, 14d6 at 8th level, 18d6 at 12th level, 22d6 at 16th level, and 26d6 at 20th level. After you use your breath weapon, it must recharge on a roll of 6 or 5 on a d6 die Additionally, the range of your breath attack increases as you level up. The range is 15 ft. at the start but increases to a 30 ft. cone at 5th level, 60 ft. cone at 12th level, and a 90 ft. cone at 20th level.

Size

Your size is currently Medium.

Unarmored Defense

The scales of an Infernal Dragon are tough, giving the player 13 + Dex Mod while in Dragon and Tiefling form. The Infernal Dragon has the ability to incorporate armor worn on it's Tiefling form into it's scales while in it's original form, taking on the AC and effects of the armor into it's scales (if the item needs attunement, it still takes an attunement slot). Armor fused in this way will unfuse with the Infernal Dragon's scales when they return to Tiefling form, allowing the armor to be discarded or replaced (unless the armor has a feature where it can not be taken off because of a curse or such).

Ability Score Increase

Increase in Strength by 2 and Intelligence by 1.

Form Shift

The ability to switch from original Infernal Dragon form to alternate Tiefling form and then back again. No time limit for both forms and unlimited use. Tiefling form looks, sizes (sizes must be within Medium range), and features can never be changed or altered unless by a spell or potion (or other ability that allows for such). 

Languages

You can speak, read, and write Infernal and Draconic.

Resistances/Immunities

Infernal Dragons keep their immunity to fire and gain a resistance to necrotic energy.

Darkvision/Blindsight

Darkvision

You have darkvision for 60 feet and it increases to 120 feet at 5th level.

Blindsight

You have blindsight for 10 feet and it increases to 30 feet at 5th level and 60 feet at 12th level.