This is Kaguya Houraisan and Fujiwara no Mokou's battle theme from the fanmade Touhou game published by LION HEART, Mystical Chain
Originals: 竹取飛翔 ～ Lunatic Princess ／ Bamboo Cutter Flight ~ Lunatic Princess (Kaguya Houraisan's theme, from 東方永夜抄 ～ Imperishable Night)
月まで届け、不死の煙 ／ Reach for the Moon, Immortal Smoke (Fujiwara no Mokou's theme, from 東方永夜抄 ～ Imperishable Night) _______________________________________________________
SO FUCKING MAJESTIC. I CAN'T EVEN.
So, you may be wondering, "Hmm, shouldn't it be Kaguya AND Mokou's theme? Do you get to play as one of these characters hmmm I am confused someone explain" okay okay calm the fuck down I'm explaining. So basically, in this battle, you are fighting Kaguya and Mokou.
Who are also fighting each other. At the same time.
... DANMAKUCEPTION *shot repeatedly*
It's literally a battle within a battle. The siren goes off, and *BOOM*, the dragon princess and the everlasting phoenix appear and dance in blood under the pale moonlight.
... and you just stand there and take shots at either one of them whenever they become vulnerable, and the other stands there and laughs. There is literally no point to this battle because they aren't even attacking you, they're attacking each other. But the music.
Oh dear God it's beautiful.