Acqua of the Back self-destructed.

The elements of the Saint and the Divine Mother reacted to the ‘Saint Destroyer’ attack, starting to fight each other and causing a chain reaction that the ‘Saint Destroyer’ shouldn’t have been able to do within his body.

The lake in the darkness of the night was surrounded by something like a flash of daylight. Kamijou and the rest could only see white, and their ears could only hear the sound of a large amount of water being boiled, an uncomfortable sound.

The moment Kanzaki Kaori opened her eyes, Acqua didn’t exist anymore.

However, the water in the artificial lake had boiled off, and the surroundings were a mess, as the pillars of steam that were as big as the surface area of the lake collided with the huge pillars and ceiling before scattering all over the place. It was like a huge 1000 year-old tree that was full of energy, showing how explosive Acqua’s explosion had been.

[...]

“You’re…still alive…?”

“Only one who attacked his enemy with the intent to kill and then gained proper evidence that he had killed the enemy should ask that question.” Acqua sighed out of annoyance. “To be honest, I did not expect you all to cause the power I had stored up by using the Virgin Mary Worship spell to explode from within. I was able to immediately create a bypass and release the mass of power outside my body, but that has caused my power to temporarily drop to the level of a normal Saint.”

Toaru Majutsu no Index vol.16, 18