The Rise of Whiro

In this game, you play as the lord of darkness, Whiro, you must traverse the underworld in the interest of devouring souls.

Whilst playing as this character, there are 3 forms that you can turn into. 

To mutate into the next form(s), you must ingest enough souls.

Once you've taken enough souls, you will be transported to earth where you can then feed on the souls of the living.