Your human nature lets you increase all your scores by one while your demonic nature allows you to choose one score to increase by an additional one.

Humans reach adulthood in their late teens and live less than a century, however, due to the fiend you can live for up to 300 years.

 Humans tend toward no particular alignment. The best and the worst are found among them. The fiend inside you however, tend to drag you towards Evil if it's able to.

Humans vary widely in height and build, from barely 5 feet to well over 6 feet tall. Regardless of your position in that range, your size is Medium.

Your base walking speed is 30 feet.

The fiend inside your soul grants you Darkvision for up to 60ft. When you're in complete darkness your eyes start to glow a low yellow glow. (similar to how cat eyes seem to glow in the dark).

Upon starting your turn with 50% of your max health points or less, you must make a DC 15 Wisdom saving throw or your inner fiend takes control of your body, creating an outer shell over your body that takes the image of a fiendish creature. (the player is free to customize the fiendish look to some degree, talk with your DM for customization, regardless of how you choose it to look there are some traits that are always there).

The fiend gains sharp claws that count as natural weapons, dealing 1d6+ strength modifier slashing damage and they count as magical for the purpose of overcoming resistance due to the nature of how these weapons came to be. The new fiendish shell also has better eyesight, allowing it to see in both non-magical complete darkness, and in magical darkness. While in this form the fiend also gains a +4 to AC as the hard shell acts as protection both for the body and the fiend.

The fiend can also cast a few spells while it is transformed, regardless of if the host has magical aptitude and ability. the spells are Fireball at third level, darkness at second level, counterspell and dispel magic at third level and inflict wounds at 5th level. The fiend has an amount of spell slots equal to your proficiency modifier.

The fiend will do its best to protect the host body for self survival and self interest, and will regard anyone as an enemy disregarding who the host body regarded as an ally or friend. While in this form you must attack the nearest target, or whoever is perceived as the highest threat to the fiends survival. (make a DC18 Perception check to see if the fiend can recognize the biggest threat (level one) or a DC14 perception check (level 5)).

As you grow stronger, so too does your control over the fiend inside you, and by the time you've reached level 8 you are strong enough to enter your inner self and do combat against the fiend inside you. The fiend has all the abilities you do, and while battling it within yourself it looks like a distorted version of you. The fiend has the same stats as the player character but with a few exceptions. The fiend is always Neutral Evil regardless of if its a demon or a devil and regardless of what your alignment is and the fiend cannot use any spells that would be considered holy in nature (Cleric and Paladin spells).

If you fail to defeat the fiend you are stuck inside your inner world and the fiend automatically takes over for 1d6 hours, during which it does not need to transform into its fiendish form unless it wishes and it looks exactly like you with the exception of your eyes, which turn into black eyes with yellow pupils.

If you defeat the fiend, you gain the power to partially transform into its form 1+con modifier times a day. When you take on this form you can choose to temporarily gain its eyesight, its claws or its armor. The eyesight takes the form of a mask covering your face, the claws appear on top of your hands acting like in place of your unarmed attacks and the armor takes the shape of a thin layer of a shell-like creation much like the hide of a dragon or any other creature that gets natural armor.

Regardless of the form you chose you can cast one of the spells the fiend knows one time per transformation without it requiring a spell slot or any components.

The duration of the transformation is decided by your constitution modifier. a +0 allows you to stay in the form for a minute, a +1 allows you to stay in the form for two minute, +2 allows you to stay in the form for three minutes, +4 allows you to stay in the form for four minutes and a +5 allows you to stay in the form for five minutes before you revert back to your own "natural" self.

After you have successfully defeated your inner fiend in your inner world the fiend is only able to take over your body if you roll a 5 or less on the wisdom saving throw (ignoring the bonuses granted by proficiency or wisdom modifier).

You can speak, read, and write Common, infernal or abyssal and one extra language of your choice. Humans typically learn the languages of other peoples they deal with, including obscure dialects. They are fond of sprinkling their speech with words borrowed from other tongues: Orc curses, Elvish musical expressions, Dwarvish military phrases, and so on.