Blackening

is a process by which a being is tainted by an outside force, causing their personality to be reversed toward the orientation of evil. For Servants, this is one of several processes that can lead to an Altered Saint Graph. Blackening can be caused by several different events.
==Holy Grail Mud (All the World's Evils)==
Heroic Spirits that come into contact with the mud from the Fuyuki Holy Grail, tainted by All the World's Evils, gain bodies of cursed flesh, and they are tainted by it as well. The contaminated servant's personality reverses toward the orientation of evil. The blackened Servant is incarnated by the power of the Grail so the more concrete ties to the physical plane turns them into powerful beings. Their physical interference power increases, but at the same time, they lose the ability to return to spirit form. They no longer bother to restrain their magic energy consumption, so their fighting ability jumps significantly. They do not have precise control, but their destructive power is increased. Their momentum can be likened to a runaway car that never runs out of fuel, so fine handling is impossible because it is out of control.
Saber being swallowed Manga.png|Saber being swallowed as the Shadow begins the process of Blackening her.
FSN Heavens Feel Film-Saber meets her corrupt self.jpg|Saber meeting Saber Alter in her mind as the latter was about to consume her.
They are fearsome beings inflicting havoc and death upon the world. These are blackened Servants like Saber, completely corrupted and changed into Saber Alter (Which Presage Flower expanded upon by showing the latter's personality meeting and consuming the former inside her mind as her physical body was being changed by the mud), while someone with an extremely strong ego like Gilgamesh was able to resist the majority of the effects. During the Heaven's Feel scenario, Archer states that distorted Heroic Spirits like himself and the True Assassin have some resistance against the tainted ichor and corrupting influence of the Holy Grail.Fate/stay night - Heaven's Feel route - Day 10: darkwood,sleepingbeauty - Distraction
During the True and False Holy Grail Wars, Bazdilot Cordelion would use Grail Mud to twist True Archer from his heroic persona into a hateful half-Avenger.Fate/strange Fake Volume 3 - Prologue VIII: "The Star Performers' Feast (Part 1)"
==Holy Grail Mud (Fate/Prototype)==
In Fate/Prototype, the Servants who were defeated in the first Holy Grail War were later revived as blackened versions of themselves via the power of Beast of 666 and the Greater Grail. In addition to being blackened, not only were their powers enhanced, their appearance became distorted (e.g. vials attached to Brynhildr's neck) and their personality underwent drastic changes. In the case of Jekyll, Hyde became the dominant personality and his appearance changed to that of his Assassin counterpart. All of the Servants also became more fierce and violently aggressive. 
==Chaos Tide==
The Sea of Life is a Personal Skill of Tiamat that expresses her Divine Authority as "the Sea From Which Life Itself is Sprung". Being that the Sea cycles the True Ether of the Genesis of Earth, the supply of Magical Energy available to Tiamat's Magical Energy Reactor is within the boundaries of the domain unlimited. The movement by which the Sea of Life extends its domain is referred to as the Chaos Tide. 
The substance of the sea is Black Mud that pollutes Saint Graphs, remaking any organisms that enters unto the sea into a "Child of Tiamat". Upon emergence from the mud, the Children of Tiamat are imposed with a cellular-level Geis that makes them into enemies of Man. Servants are no exception to this process.Fate/Grand Order - Babylonia: Absolute Demonic Front - Section 9: The Mother of Demonic Beasts
During the events in the Babylonia Singularity, Kingu mentioned that the Black Mud from the Sea of Life is the substance referred to as the Mud of the Grail amongst practitioners of Magecraft.
==Pandora's Box==
Being utilized by Darius Ainsworth, while some who are affected turn into Alters, such as Saber Alter, others are different. Some changes are quite mild, with Medusa and Medea suffering mere cosmetic changes, while Heracles' elbow spikes become much more exaggerated. In the most extreme cases, a massive physical change can occur, such as Cu Chulainn transforming into a werewolf-like shape, or Gilgamesh assuming the form of a deformed and blackened giant.Fate/kaleid liner PRISMA☆ILLYA 2wei! - Chapter 26: "Black Fate" The defining trait of those under the effect of Pandora's Box however is an all-consuming thirst for the Holy Grail as a means to fulfill their wish, making them perfect shock troops for Darius and his allies.Fate/kaleid liner PRISMA☆ILLYA 3rei!! - Chapter 22.2: "One Hand Takes, Another Hand Saves"
==Velber==
Not much is known beyond one of the Velber Anti-Cells, presumably Velber O3, being able to begin corrupting Gilgamesh from the Far Side of the Moon, turning his hand blackened briefly using "malicious information" before he was able to force it back. Gilgamesh describes the being as a weapon that corrupts the soul, and wonders if it is the original form of the "Goddess of Vengeance" or something that has just taken its shape. Artoria confuses and compares it All the World's Evils due to its ability.Fate/Extella Gilgamesh's Side Story: Shadow of the Golden King
==References==
it:Annerimento
vi:Hắc Hóa
Category:Holy Grail War terminology‎
Category:Terminology in Fate/Grand Order
Category:Terminology in Fate/stay night
Category:Terminology in Fate/Zero
Category:Terminology in Fate/kaleid liner PRISMA☆ILLYA
Category:Terminology in Fate/Prototype
Category:Terminology in Fate/strange Fake