Main female protagonist of the visual novel Saya no Uta.  ~!Saya is a being from another dimension who materialized in this universe for the sole purpose of reproduction. She has no memories of where she came from and is guided only by her instincts. Due to Fuminori's condition, he perceives her as a little young girl in a white dress, but actually, she is some sort of amorphous, tentacled, fleshy abomination that emits a putrid stench. She preys on creatures of all sizes, from cats to human beings, typically killing them by snapping their necks or disembowelment through some unknown means, and then feasting on their internal organs. Her true form is never fully revealed, as she immediately kills and consumes any third-party observers. The small amount of information regarding her actual form is gleaned entirely from in-game descriptions, which themselves are rather vague and never go into detail.!~

Parody opening for a spin-off of Mahou Shoujo Madoka Magica, starring the characters from the upcoming visual novel game SoniComi by Nitro+. 
This was released by Nitro+ as an April fools' prank, complete with an official website announcing a full TV series.

Fuminori Sakisaka, a young man who lost his family in a tragic car accident, has his life saved through an untested surgical procedure. However, thanks to a strange side-effect, all that comes upon his eyes appears twisted and ugly. All except for the beautiful girl who goes by "Saya", who draws the color of beauty to Fuminori's stained existence. For him, she is the only strand of light still remaining in life.
 
(Source: Seikaisha, translated) Note: Includes 1 extra chapter.