There was a problem with attacks that required charging. Despite what many shows and tales of glory would have you believe, one did not simply stand and stare in awe as a character prepared his or her final attack. They would do their own preparation, or, entertainment forbid, they would strike while their opponent was charging, which was another word for one thing in a battle: vulnerable. 

Gilgamesh, however, was no such coward. However, he was no fool either. He would not strike down Stein with a weapon such as Gae Bolg, which would now kill Stein due to the fact that Aqua Vitae had restored his heart - and invulnerability. However, he would not fail to make any preparations either. 

Perhaps Stein had expected Gilgamesh to pull out a multitude of treasures. Perhaps Stein expected Gilgamesh to pull out shield after shield, barrier after barrier, defence after defence. Perhaps Stein had hoped Gilgamesh would pull out enough treasures to defend a mortal from the gods, and then watch as his strike tore through those defences. However, Gilgamesh would do no such thing.

Gilgamesh pulled out only one single treasure from the gate. 

Fragarach. 

The noble phantasm and mystic code levitated in Gilgamesh's right hand. Stein should have seen what the weapon was capable of; after all, seeing it penetrate Mia's heart was what prompted him to prepare for it by removing his heart from his body - a body which had it's heart restored. Such an irony - by restoring his heart, Aqua Vitae had restored his weakness. 

[quote=LaXnyd]

"Fragarach."

And the dagger left Gilgamesh's hands. 

Fragarach was a dagger wielded by Bazett, a human. It was a dagger that would leave and would strike the enemy before he could strike first. It was a dagger that would warp the very air and space around them, rewriting reality to force it's successful strike in. Once it left, it would always strike the target in the heart - or, lacking that, their core of power. Fragarach, however, had only one condition for it's use and it was the reason why Gilgamesh never used it. Fragarach required the opponent to use their special ability. Special abilities such as the Black Blood. With Black Blood being used by Mia, Fragarach left Gilgamesh's hands. Reversing time, destiny, a trick of the gods, the very same gods Mia boasted about being the daughter of a slayer of, Fragarach pierced Mia's heart.
[/quote]

The moment Stein fired the God-Killer Resonance, Fragarach, as the retaliator, would fire. As the very concept of the weapon demanded, the very flow of time and space would be changed so that Fragarach always "hit first", and it would lodge itself in Stein's newly regrown and vulnerable heart. As it had "shot first", as it had reversed causality to strike before the enemy could, the moment Stein fired the God-Killer Resonance Fragarach would fire and return the world to the moment before Stein fired and kill him before he could fire the God-Killer resonance. 

[s]It was the ultimate Han Solo weapon. [/s]

However, the king was merciful. This weapon would only activate if Stein decided to attack. The king gave Stein a chance to back out of the strike. 

"Humph." the King said in contempt. "instead of glaring at me like you are at the moment, why don't you run along and go after those that took the girl?" he pointed out, reminding Stein of the fact that Team Rwby had taken Mia away from the room.