Fragarach

The ultimate form of counterattack, an unavoidable strike carried out in response to an enemy’s strongest ability. It both kills the enemy and nullifies their attack, creating a perfect balance between offensive and defensive capabilities. However, it far less convenient than how it may sound: It can only be used at full potential against an enemy’s most powerful strike, otherwise its power is reduced to that of a mere Rank D weapon, only ten of these can be created per year by pouring Bazett’s blood on them and letting them charge for a whole month (During Fate/hollow ataraxia she only has three shots remaining) but it is less effective against those with the ability to regenerate, such as Berserker, or Lancer's Gae Bolg - Impaling Barbed Death that pierces the target’s heart before the attack is even carried out. In the case of Berserker getting hit with Fragarach, he would lose one life unconditionally, seeing that God Hand is always active. In the case of Lancer's Gae Bolg, both Lancer and Bazett would be killed, as both attacks are unavoidable.

As rare an occurrence as it may be, Fragarach is indeed a Noble Phantasm wielded by a living human. It is one of the few remaining relics not yet attached to a Heroic Spirit.

Based on the magical sword from Irish mythology, said to be forged by the gods. Known as 'The Answerer' or 'The Retaliator', it passed through the hands of more than one hero, including Cú Chulainn.