I don't post here often, but I don't know if this Sailor Moon series was posted yet and so I'm taking a shot (very extreme hentai involved).
Dark Planet Syndrome (Hozumi Touzi, TKS)
http://g.e-hentai.org/g/596726/153b3c42b4/
http://g.e-hentai.org/g/646097/f7a753c785/
http://g.e-hentai.org/g/695085/ce6b567627/
http://g.e-hentai.org/g/818612/48c307e6a4/
It starts off fairly tame, but the transformations/corruptions get more and more insane and graphic with each book. Only Part 1 has a translation to English, though, and I would love to see the rest done sometime.
http://g.e-hentai.org/g/603050/2d1e26d34c/