Warriors corrupted by the lower planes, touched by the primordial chaos of the abyss, the abyssal knights are the traitors of the material plane, acting as agents of destruction to their demon overlords. These fighters are often power-hungry individuals, who have forfeited their souls in exchange for demonic abilities. Some do so to accomplish some mundane goal, believing that these profane powers will allow them to conquer glory or gold; others, however, do it for a more nefarious reason, expecting to fight in the blood war as generals, reincarnated as a Balor after their deaths. The most ignorant and foolish of them think they can master this dark path, unaware that even the most hardened or pure-hearted warrior eventually succumbs to evil, madness, or sometimes both.

(Description and inspiration for this subclass taken from D&D Wiki's Abyssal Knight subclass)

Bonus Proficiencies

3rd-level Rune Knight feature

You learn to speak, read, and write Abyssal.  In addition, you have advantage in all your Wisdom (Insight) and Charisma (Persuasion, Deception, and Performance) checks made in relation to demons.

Abyssal Step

3rd-level Abyssal Warrior feature

At 3rd level, when you choose this archetype, you gain the ability to teleport short distances by taking a shortcut trough the lower planes. As an action, you can teleport to a place you can see within 30 feet.

When you do so, flames erupt from the place you just left, and all creatures in a 5-foot radius must succeed on a Dexterity saving throw (DC 8 + your proficiency bonus + your Constitution modifier) or take 1d6 fire damage. This increases to 2d6 at 5th level, 3d6 at 11th level and 4d6 at 17th level.

You may do this a number of times equal to your proficiency bonus, regaining all uses at the end of a long rest.

Demonic Weapon

3rd-level Abyssal Knight feature

At the end of a long rest, you can imbue up to two weapons of your choice that are on your person with unholy power. Each time you deal damage with those weapons, you deal an additional 1d4 fire damage. This extra damage increases to 1d6 at level 11.

Fiendish Senses

7th-level Rune Knight feature

At 7th level, you gain fiendish eyes that grant you the ability to see through normal and magical darkness up to a range of 120 feet.

In addition, you can communicate telepathically with any creature that can understand a language, up to a range of 30 feet.

Fiendish Form

10th-level Abyssal Warrior feature

At 10th level, your features and skin change to resemble that of a demon. You gain resistance to fire and poison damage, and advantage on saving throws against the poisoned condition. Additionally, you can use an action to intimidate a foe. Make a Charisma (Intimidation) check against a creature, contested by that creature's Wisdom saving throw. If the target fails the saving throw, it is frightened by you for one round.

Demonic WIngs

15th-level Abyssal Warrior feature

Starting at 15th level, you grow a pair of leathery wings, that grant you a flying speed equal to your movement speed. You can reveal or hide these wings as an action, and you cannot benefit from them while they are hidden. If the wings are active, you must be using armor specifically crafted to accommodate these wings. Armor can also be adapted to fit these wings for 10gp for each tier of rarity (starting at 10gp for common).

Fiery Aura

18th-level Abyssal Warrior feature

When you reach the 18th level, you can use your bonus action to activate an aura of fire. At start of your turn, any creature within 5 feet takes fire damage equal to your Constitution modifier (minimum of 1 damage). In addition, any flammable objects in the aura that aren't being worn or carried ignite.

The aura lasts until you're incapacitated or you dismiss it as a bonus action. Once dismissed, you can't summon this aura again until you finish a short or a long rest.