After seeing the article on Malcanthet, I can add a few things on DnD.
In Fiendish Codex I: Hordes of the Abyss, there are expanded rules for demonic possession. In The Book of Vile Darkness, they had the usual 'possess people and make them do evil things' sort of possession. Fiendish Codex I adds the option for the demon to actually transform the host body into the Demon's. The end result has all the powers of the demon. There's even a short process in the book, the two pics can be found here:
http://www.wizards.com/dnd/images/fc1_gallery/98437.jpg
http://www.wizards.com/dnd/images/fc1_gallery/98436.jpg
Every day, the victim of the possession makes a saving throw. If it fails, one quarter of the victim turns into the Demon. Once all four saves are failed, the demon takes over completely. It's left up to the DM and players what 'one quarter' means (am I gradually turning into a succubus or do I now have a sexy leg?).
There are also numerous references in the books that powerful Demons have the ability to convert mortals into half-fiends, which changes the character's alignment to evil.
If that's not enough, there's a spell in the sealed section of one of the Dragon magazines that impregnates a victim, the fetus grows very quickly then is born as a half-fiend version of the victim.