Yousei Teikoku

I am the fairy.

Yousei Teikoku, literally meaning "The Fairy Empire", is a techno/trance/gothic/heavy rock/light metal/classical/symphonic/dark ambient Japanese band who got their start in the 90's. Their songs have been featured in several anime and games including Venus Versus Virus, Kurokami, Mai Hi ME, Queens Blade, Seikon no Qwaser, Innocent Venus, and Magical Pokaan.

As quoted (and translated roughly) from their official website:

"The belief modern humans have in fairies has lessened over the years. with our music, we aim for the reconstruction of the fairy world devastated by this lack of belief."

Lineup includes (with the latter two added this year):

Yousei Yui (real name Yui Itsuki) - Vocals, chorus, lyric-writing.
Takaha Tachibana - Guitar, keyboard, composition/arrangement, lyric-writing.
Nanami - Bass.
Relu - Drums.

And their discography includes:

Atarashii Momo (1996)
Momo no Hane (1997)
Momo no Mori (1998)
Shikou no Momo (1999) [remix album of the previous Momo's]
Owari no Shissu ~Tsui no Yakata~ (2004) [ending songs compilation]
Stigma (2005)
Mai-Hi ME OST - Last Moment (2005)
Ashita wo Yurushite (2006)
Senketsu no Chikai (2006)
Noble Roar (2006)
Valkyrja (2006)
Shijun no Zankoku (2007)
Gothic Lolita Propaganda (2007) [and their first non-indie release]
Metanoia (2007)
Schwarzer Sarg (2008)
Hades-The Bloody Rage (2008)
Weiß Flügel (2008)
Irodori no Nai Sekai (2009)
Gekkou no Chigiri (2009)
Gothic Lolita Doctrine (2009)
One (2009)
Baptize (2010)
Rebellion Anthem (2010)
Asgard (2010)
Gothic Lolita Agitator (2010)
Mischievous of Alice (2011)
Kuusou Mesorogii (2011)
filament (2012)

Yousei Teikoku provides examples of:

Ascend to A Higher Plane of Existence: It's generally agreed Schwarzer Sarg is about death. Though depending on what translation you're looking at, it also tells a tale of a girl who died (possibly by suicide) and looks down and pities the rest of the human world. That, or it's just Word Salad Lyrics about death.
Ascended Fangirl: Yui voiced a character in Black Butler.
Corrupt the Cutie: Arguably, Shijun no Zankoku.
Deal With the Devil: Kikai Shoujo Gensou and Senketsu no Chikai.
Elegant Gothic Lolita: Yui, oh so very much. Just look at the two main album titles.
Exactly What It Says On the Tin: Kikai Shoujo Gensou means The Machine Girl's Fantasy
For Doom the Bell Tolls: The beginning of Wahrheit.
Gratuitous English: In Patriot Anthem: "I am a martyr! I am a believer! I believe just in the fairy!"
Gratuitous French: Eternal Waltz.
Gratuitous German: And HOW!
- Just to drive the point home, they had an official live called 920Putsch.
Ill Girl: The protagonist of Kikai Shoujo Gensou.
Keep Circulating the Tapes: Again, their old indie albums (before Stigma). Recently, someone found (albeit a very low quality) Momo no Hane and Momo no Mori, but Shikou no Momo and Atarashii Momo are still nowhere to be found.
Les Yay: The video for Mischievous of Alice. So much.
Lighter and Fluffier: Their old indie albums. Moonlight Magic, yeah...
- And the more recent Irodori no Nai Sekai and Ashita wo Yurushite.
Lonely Piano Piece: Weiß Flügel fades into one near the end.
Meaningful Name: Hidden Truth is a purely music box version of Wahrheit, which not many people seem to realize. Wahrheit means "truth" in German.
Mondegreen: The chanting near the end of Kikai Shoujo Gensou. Despite it being in Japanese, It's written in katakana in the lyrics book (which is the alphabet used almost exclusively for foreign words). Which makes translating quite difficult.
Music Box Intervals: Wahrheit.
Neoclassical Punk Zydeco Rockabilly: See above. The "genres" section of their Wikipedia page changes about twice a week. Yeah.
Non Appearing Title: Nearly all of their songs. One of the only ones that does appear is Kokou no Sousei.
Oh Crap: Basically the reaction of Kikai Shoujo Gensou 's protagonist as she realizes that getting this "perfect" machine body will take away her humanity.
Ominous Latin Chanting: Too many to list.
Ominous Music Box Tune: Hidden Truth.
Only Known By Their Nickname: Averted with Yui. Even though she officially goes by "Yousei Yui" in the band, she's also a seiyuu, and it's pretty well known that her last name is Itsuki.
Really 700 Years Old: Yui, "as a fairy", claims to be 4325 years old.
Robot Girl: What the aforementioned Ill Girl becomes.
Scare Chord: Wisdom starts out as a creepy little piano tune, then...BAM.
Signature Song: Last Moment or Kokou no Sousei.
The Stoic: Yui pulls this off very well.
Summoning Ritual: Senketsu no Chikai.
Ye Olde Butcherede Japanese: Again, this is merely whatever translation you're reading, but...Senketsu no Chikai: "O mysterious spell of summoning, bequeath thy answer to me."
Zettai Ryouiki: Yui on the cover of Stigma.)