Appears in the Type-Moon [[April Fools]] joke [[Rojiura Satsuki: Chapter Heroine Sanctuary]], [[Fate/Grand Order]] and [[Fate/EXTELLA]].

Yet another version of [[Artoria Pendragon (fate)|]], this time summoned as the Assassin Class. Wears a [[scarf]], [[blue jacket|blue]] [[track jacket]], [[shorts]], and a [[baseball cap]] that somehow fails to conceal Artoria's iconic [[ahoge]].

[spoiler] Her goal is to rid the universe of the ever growing Artorias and their ilk as she considers them a plague upon the universe and once they are gone she shall be the one and only Artoria. Although primarily focused on her mission to exterminate all Artorias, she has opened her heart to [[Saber Lily]].

Consequently, Artoria wears Mysterious Heroine X's outfit as an alternate costume in Fate/EXTELLA.[/spoiler]

Wields the mysterious magical sword, the [[Himitsucalibur (fate)|]]. Its capabilities and powers are a secret.

h4.Costumes

* [[mysterious heroine x (third ascension) (fate)|Third Ascension]]

h4. Variants

* [[Mysterious Heroine X Alter (fate)|]]
** [[Mysterious Idol X Alter (fate)|]]
* [[Mysterious Heroine XX (Fate)|]]
* [[Ultra Heroine Z (fate)|]]

h4.Costumes

* [[mysterious heroine x (third ascension) (fate)|Third Ascension]]
謎のヒロインX ヒロインX