You're listening to an awesome, high energy track from the Mai HiME soundtrack. This noteworthy BGM accompanies the appearance of Natsuki and her Child "Duran". While the name of her Child is in the title, it's more of a entrance theme for Natsuki to me. The song itself reminds me of something from Noir or Madlax. Oh wait, how silly of me. Yuki Kajiura was responsible for this masterpiece. She has a tendency to be involved with the greatest franchises in history. The awesomeness of the techno-esque beats. When you hear the howling, you know you're gonna get your ass kicked. ;)
Natsuki Kuga is my favorite character in the series. She's beautiful and kick ass, yet so tragic. In a series where it's biggest message (to me anyway) being "Love Transcends Everything" (which includes barriers even that were gender related), Natsuki shines the brightest. More so than Mai, and Mai is more 3-dimensional than fans give her credit for.