Theoretical fan-designed dark persona version of a character who has been corrupted by the mud of the Fuyuki Holy Grail from Fate/Stay Night, similar to canon examples like Dark Sakura or Saber Alter. Fan examples of "alters" include Archer Alter, Lancer Alter, or the "Sabatsugu" version of Emiya Kiritsugu (not to be confused with Emiya Kiritsugu (Assassin)).
Common traits of an Alter version of a character include white hair, red eyes or yellow eyes, pale skin, and visible red veins of corruption. Alter characters will also typically be dressed in an alternate costume with darker colors or stripes of black and red.