This character is subjected to a brand of Sexual Corruption, which results in them eventually switching sides from the side of Good, to Evil. Just like with other forms of Sexual Corruption, the character starts the game as pure/just/good, and is subjected to magic, mind control, rape & mindbreak, or other such techniques, that eventually make the character love/lust their corrupter and join their side.

As part of their fall, the character acquires a new form which represents an evil or corrupted version of themselves. This form needs to have a different appearance compared to the original form the character had.

The new corrupted form could be either the result of Mahou Shoujo doing a Henshin (transformation) into a lewder version of herself, or it could be a permanent form achieved through training (e.g. Venus Blood franchise).

For the trait to apply:
1) The game must be set in a universe with clearly defined sides of Good and Evil. If story is too complex and has shades of Grey, or if the Good/Evil dichotomy is not clearly defined, then the trait should not be used.
2) The character must change sides from non-Evil to Evil, achieved through sexual corruption perpetrated by the Evil side.

Do not confuse the trait with Evil Form, which represents evil forms and transformations acquired through evil deeds of the character.

From Example A to Example B