Hito Hito no Mi, Model: Demon

The Hito Hito no Mi, Model: Demon is a Mythical Zoan-type Devil Fruit that allows its user to transform into a demon hybrid and a full demon at will, making the user a Devil Human (悪魔の人間). It was eaten by the doctor of the Blacksmith Pirates, Mary Wolverston.

Strengths
Like most Zoans, this fruit grants it's user the ability to transform between three forms. The first is that of their normal form. The second is a hybrid form, a combination between their original state, and a  demon. The third and final form is the ability to transform into a fullly fledged demon, with both the full and hybrid form granting the consumer an extreme boost in physical attributes such as speed, strength, and the ability to generate and manipulate the Flames of Hell.

This ability allows them to control special black flames that cannot be put out unless the user wills them to and leaves scars that are engraved in the person genome - meaning the scar is passed on from generation to generation. The Flames does not operate like normal fire and is intangible, it is able to harm Logia and is resistant to cold or any other thing that would normally put out a fire.

The Zoan Fruit, unlike most, actually has more than one transformation form that not only increases the physical attributes and healing factor from each new form, but also grants new abilities.

Weaknesses
This fruit while powerful, is not without its weaknesses. The fruit is subject to all normal Devil Fruit weakness of course, but is also highlighted by one of thing which has been dubbed The Devil's Curse. In return for such immense power and versatile abilities not only does the user feel an insane amount of pain during and after the transformation it gradually turns the user in a demon permanently after prolonged use. The most chilling thing about this is the fact that when permernantly turned into a demon the user does not lose there mind, but later go insane from their own grotesque body.