Blackening

Blackening (黒化) is a process by which a being is tainted by an outside force, causing their personality to be reversed toward the orientation of evil. There are several different processes that can blacken a being.

Holy Grail Mud (All the World's Evils)
Heroic Spirits that come into contact 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 process is called .<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, but fine handling is now impossible because it is out of control.

They are fearful beings inflicting havoc and death upon the world. These are blackened Servants like Saber, completely corrupted and changed into Saber Alter, 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.