You know a list of Infernal spells innate into your very being depending upon your level you have access to more or less of these spells of which you regain the ability to cast when you finish a long rest. Charisma is your spellcasting ability for these spells.

However, each time one of these spells are used you must role a Wisdom saving throw (DC 13), on a failed save you are overtaken by your strong, powerful infernal blood causing you to assume your Demonic Form.

Your high infernal parent’s blood burns through your veins releasing the inner infernal nature you possess, exposing your infernal form as the son of the Betrayer.

When in this form:

Your alignment immediately becomes Chaotic Evil.

You attack whoever is closest upon your turn or if attacked will target the last to hit you.

Hit Points are increased by 30% temporarily

You are Vulnerable to Radiant damage and Psychic damage. 

You are Immune to Fire damage and Frightened.

You are Resistant to Non-magical attacks and Necrotic damage.

You gain Advantage in Intimidation providing that the target is less than 7 levels above you.

If you are grappled the one grappling you must roll a constitution saving throw every round when grappling. On a fail they take a 1D10 on a success they take half

Fiery Step

You gain the use of Misty Step usable twice per form change. Your form combusts spontaneously and the embers of your body float on the wind and settle in a desired location within view 30 feet away where you quickly reanimate from the embers.

Ability Score Increase. Increase your Strength or Dexterity and Charisma score by 4, to a maximum of 30.

Overcome Defenses. The Bludgeoning, Piercing, and Slashing damage you deal always ignores Resistance.

Overwhelming Strike. When you roll a 20 on the d20 for an attack roll, you can deal extra damage to the target equal to the ability score increased by this feat.

The extra damage's type is the same as the attack's type.

Ancestral Hate:

Despite your powerful form the inner human side of you despises it causing an inner fury each time you take advantage of the form’s abilities. If you use anything other than a standard attack at the end of your turn you take 1D12 psychic damage (adding 1D12 at level 5,7,9,11,13,15,17,19)  and 1d4 for every attack used during the turn over one. If this leads to death you return to your standard form unconscious (until 10th level where you have 5hp) and gain a level of exhaustion. 

After each round you must also make a wisdom saving throw (DC16) if the human you would like to break free. On a success you return to human form with two points of exhaustion and half of your current hit points. On a failure the Infernal form remains.

Call Upon the Human:

If a character the DM considers emotionally entwined with the you attempts to talk you down they can roll a persuasion check against a constitution saving throw of yours to try and release you from the Infernal form. On their success you are released from the form with half of your current Hit Points and a level of exhaustion. On your success their attempts are ignored. If they use a charm spell or another emotionally entwined person to help with this endeavor they gain advantage. However after three attempts your mind is closed to their words and can no longer be saved by them.

Infernal Feed:

Starting at level 5, each round you gain 10% of the damage you deal as hit points. This increases by 10% at level (8,11,14,17,20)

Infernal Flight:

Starting at level 3 you gain a flight speed of 15 feet while in this form. This increases by 5 feet at levels (5,7,9,12,15,18,20)

Infernal Invocations:

Level 5:

Fiendish Vigor:

You can cast False Life on yourself without expending a spell slot. When you cast the spell with this feature, you don't roll the die for the Temporary Hit Points; you automatically get the highest number on the die.

Devil's Sight:

You can see normally in Dim Light and Darkness-both magical and nonmagical -within 120 feet of yourself.

Thief of Five Fates:

You can cast bane once using a warlock spell slot. You can't do so again until you finish a long rest.

Sign of Ill Omen:

You can cast bestow curse once using a warlock spell slot. You can't do so again until you finish a long rest.

Level 7:

Dreadful Word

You can cast confusion once using a warlock spell slot.

You can't do so again until you finish a long rest.

Bewitching Whispers

You can cast compulsion once using a warlock spell slot. You can't do so again until you finish a long rest.

Sculptor of Flesh

You can cast polymorph once using a warlock spell slot.

You can't do so again until you finish a long rest.

Investiture of Flame

Flames race across your body, shedding bright light in a

30-foot radius and dim light for an additional 30 feet for the spell's duration. The flames don't harm you. Until the spell ends, you gain the following benefits:

* You are immune to fire damage and have resistance to cold damage.

* Any creature that moves within 5 feet of you for the first time on a turn or ends its turn there takes 1d10 fire damage.

* You can use your action to create a line of fire 15 feet long and 5 feet wide extending from you in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 4d8 fire damage on a failed save, or half as much damage on a successful one.

Circle of Death

Negative energy ripples out in a 60-foot-radius Sphere from a point you choose within range. 

Each creature in that area makes a Constitution saving throw, taking 8d8

Necrotic damage on a failed save or half as much damage on a successful one.

Using a Higher-Level Spell Slot. The damage increases by 2d8 for each spell slot level above 6.

Eyebite

For the spell's duration, your eyes become an inky void imbued with dread power.

One creature of your choice within 60 feet of you that you can see must succeed on a Wisdom saving throw or be affected by one of the following effects of your choice for the duration.

On each of your turns until the spell ends, you can use your action to target another creature but can't target a creature again if it has succeeded on a saving throw against this casting of eyebite.

Asleep. 

The target falls unconscious. It wakes up if it takes any damage or if another creature uses its action to shake the sleeper awake.

Panicked. 

The target is frightened of you. On each of its turns, the frightened creature must take the Dash action and move away from you by the safest and shortest available route, unless there is nowhere to move. If the target moves to a place at least 60 feet away from you where it can no longer see you, this effect ends.

Sickened. 

The target has disadvantage on attack rolls and ability checks. At the end of each of its turns, it can make another Wisdom saving throw. If it succeeds, the effect ends.

Mass Suggestion

You suggest a course of activity (limited to a sentence or two and magically influence up to twelve creatures of your choice that you can see within range and that can hear and understand you.

Creatures that can't be charmed are immune to this effect. The suggestion must be worded in such a manner as to make the course of action sound reasonable. Asking the creature to stab itself, throw itself onto a spear, immolate itself, or do some other obviously harmful act automatically negates the effect of the spell.

Each target must make a Wisdom saving throw. On a failed save, it pursues the course of action you described to the best of its ability. The suggested course of action can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do.

You can also specify conditions that will trigger a special activity during the duration. For example, you might suggest that a group of soldiers give all their money to the first beggar they meet. If the condition isn't met before the spell ends, the activity isn't performed.

If you or any of your companions damage a creature affected by this spell, the spell ends for that creature.

At Higher Levels. When you cast this spell using a 7th-level spell slot, the duration is 10 days. When you use an 8th-level spell slot, the duration is 30 days. When you use a 9th-level spell slot, the duration is a year and a day.

Level 9:

Ascendant Step

You can cast levitate on yourself at will, without expending a spell slot or material components.

Beguiling Influence

You gain proficiency in the Deception and Persuasion skills.

Minions of Chaos

You can cast conjure elemental once using a warlock spell slot. You can't do so again until you finish a long rest.

Level 15:

Witch Sight

You can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of you and within line of sight.

Master of Myriad Forms

You can cast alter self at will, without expending a spell slot.

Visions of Distant Realms

You can cast arcane eye at will, without expending a spell slot.

Finger of Death

You unleash negative energy toward a creature you can see within range. The target makes a Constitution saving throw, taking 7d8 + 30 Necrotic damage on a failed save or half as much damage on a successful one.

A Humanoid killed by this spell rises at the start of your next turn as a Zombie that follows your verbal orders.

Forcecage

An immobile, Invisible, Cube-shaped prison composed of magical force springs into existence around an area you choose within range. The prison can be a cage or a solid box, as you choose.

A prison in the shape of a cage can be up to 20 feet on a side and is made from 1/2-inch diameter bars spaced 1/2 inch apart. A prison in the shape of a box can be up to 10 feet on a side, creating a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area.

When you cast the spell, any creature that is completely inside the cage's area is trapped. Creatures only partially within the area, or those too large to fit inside it, are pushed away from the center of the area until they are completely outside it.

A creature inside the cage can't leave it by nonmagical means. If the creature tries to use teleportation or interplanar travel to leave, it must first make a Charisma saving throw. On a successful save, the creature can use that magic to exit the cage. On a failed save, the creature doesn't exit the cage and wastes the spell or effect. The cage also extends into the Ethereal Plane, blocking ethereal travel.

This spell can't be dispelled by Dispel Magic.

Hurl Through Hell:

Once per turn when you hit a creature with an attack roll, you can try to instantly transport the target through the Lower Planes. 

The target must succeed on a Charisma saving throw against your spell save DC, or the target disappears and hurtles through a nightmare landscape.

The target takes 8d10 Psychic damage if it isn't a Fiend, and it has the Incapacitated condition until the end of your next turn, when it returns to the space it previously occupied or the nearest unoccupied space.

Dominate Monster

One creature you can see within range must succeed on a Wisdom saving throw or have the Charmed condition for the duration. The target has Advantage on the save if you or your allies are fighting it. Whenever the target takes damage, it repeats the save, ending the spell on itself on a success.

You have a telepathic link with the Charmed target while the two of you are on the same plane of existence. On your turn, you can use this link to issue commands to the target (no action required), such as "Attack that creature," "Move over there," or "Fetch that object." The target does its best to obey on its turn. If it completes an order and doesn't receive further direction from you, it acts and moves as it likes, focusing on protecting itself.

You can command the target to take a Reaction but must take your own Reaction to do so.

Using a Higher-Level Spell Slot. Your Concentration can last longer with a level 9 spell slot (up to 8 hours).

Glibness

Until the spell ends, when you make a Charisma check, you can replace the number you roll with a 15.

Additionally, no matter what you say, magic that would determine if you are telling the truth indicates that you are being truthful.

Power Word Stun

You overwhelm the mind of one creature you can see within range. If the target has 150 Hit Points or fewer, it has the Stunned condition. Otherwise, its Speed is 0 until the start of your next turn.

The Stunned target makes a Constitution saving throw at the end of each of its turns, ending the condition on itself on a success.

Level 17:

Imprisonment

You create a magical restraint to hold a creature that you can see within range. The target must make a Wisdom saving throw. On a successful save, the target is unaffected, and it is immune to this spell for the next 24 hours. On a failed save, the target is imprisoned.

While imprisoned, the target doesn't need to breathe, eat, or drink, and it doesn't age. Divination spells can't locate or perceive the imprisoned target, and the target can't teleport.

Until the spell ends, the target is also affected by one of the following effects of your choice:

Burial. 

The target is entombed beneath the earth in a hollow globe of magical force that is just large enough to contain the target. Nothing can pass into or out of the globe.

Chaining. 

Chains firmly rooted in the ground hold the target in place. The target has the Restrained condition and can't be moved by any means.

Hedged Prison. 

The target is trapped in a demiplane that is warded against teleportation and planar travel.The demiplane is your choice of a labyrinth, a cage, a tower, or the like.

Minimus Containment. 

The target becomes 1 inch tall and is trapped inside an indestructible gemstone or a similar object. Light can pass through the gemstone (allowing the target to see out and other creatures to see in), but nothing else can pass through by any means.

Slumber. 

The target has the Unconscious condition and can't be awoken.

Ending the Spell. 

When you cast the spell, specify a trigger that will end it. The trigger can be as simple or as elaborate as you choose, but the DM must agree that it has a high likelihood of happening within the next decade. The trigger must be an observable action, such as someone making a particular offering at the temple of your god, saving your true love, or defeating a specific monster.

A Dispel Magic spell can end the spell only if it is cast with a level 9 spell slot, targeting either the prison or the component used to create it.

Power Word Kill

You compel one creature you can see within range to die. If the target has 100 Hit Points or fewer, it dies.

Otherwise, it takes 12d12 Psychic damage.

Weird

You try to create illusory terrors in others' minds. Each creature of your choice in a 30-foot-radius Sphere centered on a point within range makes a Wisdom saving throw. On a failed save, a target takes 10d10 Psychic damage and has the Frightened condition for the duration. On a successful save, a target takes half as much damage only.

A Frightened target makes a Wisdom saving throw at the end of each of its turns. On a failed save, it takes 5d10 Psychic damage. On a successful save, the spell ends on that target.

You have access to a wide range of innate infernal spellcasting:

Level 3:

• Fire Bolt

• Produce Flame
• Burning Hands
• Chromatic Orb
• Hellish Rebuke
• Searing Smite
• Flame Blade
• Flaming Sphere
• Heat Metal
• Scorching Ray
• Fireball
• Chill Touch
• Hex
• Inflict Wounds
• Vampiric Touch
• Poison Spray
• Vicious Mockery
• Bane
• Charm Person
• Command
• False life
• Hideous Laughter
• Inflict Wounds
• Ray of Sickness
• Sleep
• Blindness/deafness
• Enthrall
• Hold person
• Ray of Enfeeblement
• Suggestion
• Zone of Truth
• Animate Dead
• Bestow Curse

Level 5:

• Fire Shield
• Wall of Fire
• Flame Strike
• Blight
• Contagion
• Charm Monster
• Compulsion
• Confusion
• Dominate Beast
• Dominate Person
• Hold Monster
• Modify Memory
• Reincarnate

Level 7:

• Delayed Blast Fireball
• Fire Storm
• Prismatic Spray
• Harm

Level 9:

• Circle of Death
• Create Undead

Level 11:

Level 13:

• Resurrection
• Befuddlement

Level 15:

• Incendiary Cloud
• Meteor Swarm
• Clone
• Dominate Monster
• Feeblemind
• Glibness

Level 17:

• Prismatic Wall
• Mass Suggestion
• True Resurrection

Your Dexterity score increases by 2, and your Wisdom score increases by 1.

Undetermined life expectancy however they seem to physically stay at 30 for much longer than humans

Generally undetermined in alignment but if they give in they will fall into chaotic evil

Typically same as humans

Your base walking speed is 35 feet.

You can speak, read, and write Common, Abyssal, and Infernal.

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 90 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern colour in darkness, only shades of gold.

You have immune to fire damage and resistant  to necrotic.