The Nine Wrath's

Description
The Nine Wrath's Martial Arts, Also known as Nine Wrath's Essence, is as old as the 6 great golden martial arts. This forbidden book, that even the author didn't know the purpose of, was created in zongyuan during the xia dynasty. It was assembled as a colection of spells, rather than a collection of martial arts techniques. Later, when the xia dynasty started to fade away, the nine wrath essence's trail was concealed. After countless years, a scholar discovered a copy of it by chance, translated a part of it, and gained a portion of it's power. It was a technique that borrowed strength from beings in hell. Hence, the name, the nine wrath's martial arts. The nine wrath's martial arts is a simple technique. If it is practiced, the person's physical abilities multiply. They will gain herculean strength and amazing regeneration skills - the skills of the great devil in hell. But, that's only temporary. In less than a month, the practitioner's body completley transforms. No, transforming isn't quite accurate. It's more fitting to say that the person's species changes entirely. Strength is given, but the practitioner begins to resemble a devil. Finally, during the last stage of the transformation, they turn into a devil entirely. Their humanity slowly starts to disappear. It's similar to practicing evil martial arts. Eventually, the practitioner becomes inhuman. There are no exceptions.

The lord of the dark heaven's humanity is evidence that he didn't practice the nine wrath's martial srts, and only used it as a recruitment bait.