The method of transformation of devils into Rakshasa is a closely guarded secret of the Nine Hells - forming a special branch of the infernal hierarchy to infiltrate and corrupt the mortal world. Through pact, bargain or special research into the ancient and closely guarded secret, you have begun the transformation as a mortal, tasting a small measure of the Rakshasa's power as it slowly corrupts the body and soul. Your unearned transformation exposes you to great peril, should devils claim your soul before your transformation is complete you will suffer and die in the Blood Wars as a useless pawn, or worse, spend eternity as a lesser servitor devil.

Infernal Mark

At 1st level, you can speak, read, and write Infernal. Fiends recognize the infernal mark upon your soul, and that you remain mortal, making you a potential asset of the Baatezu in the nigh-eternal Blood Wars, and potentially more useful dead than alive. Whenever you make a Charisma check when interacting with Fiends, you have disadvantage on the check.

Feline Features

As an unnatural being of magically combined mortal flesh, immortal soul, living tiger and undying devil, your body changes to resemble what you are becoming. Your skin has begun to grow a coarse coat of cat fur, your hands invert, with palm outward and fingers curling away from your body with retractable claws hidden in furry folds, and magic itself bends to your otherworldly wiles.

You learn the Mage Hand and Minor Illusion cantrips in addition to any you already know. You can make a claw attack dealing 1d6 damage plus your Strength modifier in slashing damage. You have advantage on saving throws against all spells and magical effects. You gain a bonus to your Natural Armor Class equal to half your proficiency modifier. 

Burgeoning Diabolical Magic

The magical talents of the Rakshasa are growing ever nearer the grasp of your backward fingers. At 6th level, you can cast Disguise Self at will. Additionally, you can cast charm person, detect magic, detect thoughts, invisibility, and suggestion without material components at their lowest level once each, without expending a spell slot. You regain the ability to cast these spells after a long rest and these spells are always known to you if you choose to cast them using spell slots. 

Reformed Reality

At 14th level, in your natural form, you are to all outward appearances a Rakshasa. You move like like a great jungle cat, your fur hide repels mundane attacks and magical energies, your claw attacks are effortless and natural. Most importantly, your interactions with Fiends have completely changed - fiends now recognize you as an asset, a potential ally or source of information worthy of grudging respect, if not admiration. 

Your walking speed increases to 40 feet. You are resistant to non-magical bludgeoning, piercing and slashing damage. You are immune to 3rd level and lower spells, unless you choose to allow their effect on you. When you take the attack action with your claws, you make two attacks and may use either your Strength or Dexterity modifiers for Attack Rolls and Damage.
You have advantage on Charisma-based checks when interacting with Fiends.

Fiend Reborn

At 18th level, you have achieved full transformation into the magical amalgamation of humanoid, tiger and devil - A Rakshasa. You are immune to nonmagical bludgeoning, piercing and slashing damage. You are vulnerable to magical piercing damage. You are immune to spells of 6th level or lower unless you choose to be affected. You can cast Fly, Major Image, Dominate Person, Plane Shift and True Seeing without material components at their lowest level without expending a spell slot. You regain the use of this ability after a long rest. You can cast Detect Thoughts at will.

Your claw attacks inflict vile wounds which resist natural and magical healing. After successfully damaging a creature with your claw attack, you prevent the target from regaining hit points by any means until the start of your next turn.