What is the name of the opera that is sung in the start screen of "D2"
and could you perhaps tell me more about it, such as its lyrics,
translation into english, its history (if any).

For those not familiar, "D2" is a survivor/horror video game for the
Sega Dreamcast released in North America in 2000.  It was written,
produced, and directed by Kenji Eno, who also directed and composed
all of the in-game music.  About the only hint of who actually the
mysterious song is from the "D2 Original Soundtrack" with Kenji Eno
and Azusa Dodo listed as both doing "vocalisation".