Seeing Enkidu tainted like that was something that Gilgamesh could not forgive. Seeing his best friend shattered - while he knew that it was not permanently gone, as one other had, the fact remained that this mongrel had dared to touch him, dared to strike him, dared to attack him with weapons from his own treasury and finally dared to shatter the reminder of his best friend, Gilgamesh decided. 

There would be no more mercy. There would only be death. 

"...very well..." he hissed as Mia made her taunts, with the heroic spirit beginning to stand, his wounds already healing due to the properties of Avalon. This was no longer roaring, defied tyrant Gilgamesh. This was Gilgamesh when he was truly furious. "If you wish harm from me, I shall grant it." He didn't call her mongrel. She was no longer a mongrel before him. She was an insult. She was something that the king had personally decided to eliminate. Standing once again, Gilgamesh recalled all the weapons into Gate of Babylon to prevent Mia from touching his collection any further. And with that, Gilgamesh extracted what seemed like a small dagger, it's hilt concealed by Gilgamesh's grasp. 

"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.