La Pucelle

Crimson Holy Maiden. The suicide-type Noble Phantasm used by Ruler. It is a Conceptual Weapon that crystalizes one's mental landscape. It uses the Sword of St-Catherine, which Ruler never swung once in her lifetime, as its catalyst, and manifests the flames which once burned her to death. By using this Noble Phantasm, Ruler loses her own life. But in exchange for her life, she can annihilate anything in existence.
Its designated rank of C refers only to its pre-activated state while the sword is still sheathed. The EX rank after activating is because its pure destructive power cannot be calculated. These flames only destroy that which she thinks must be destroyed. In other words, this Noble Phantasm cannot be used against simply a strong enemy or out of emotions like hate.
These flames exist in order to save something, and in the novels she decided to use this Noble Phantasm out of her sense of duty to save the future of humanity.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia