The curse of All the World's Evil shown to Shirou Emiya
Angra Mainyu: All the World's Evil (この世全ての悪, Kono Yo Subete no Aku?) is a curse that had put all of the evils in the world on one person, that who became Angra Mainyu. It is not a very useful ability, as it only grants the Servant's soul an alignment directed towards killing people even after when it takes on the form of directionless and alignmentless magical energy after losing the shell of its class. Normally it would not affect anything, but with the way the Great Grail is set up to take the Servant's souls means placing "All the World's Evil" into a wish granting machine.
After Avenger is absorbed during the Third Holy Grail War, the Grail is transformed into an incarnation of Angra Mainyu that materializes as black mud and later a large humanoid under certain conditions. The materialization of Angra Mainyu brought forth by truly opening the Holy Grail is an ichor that will consume every human being, 6 billion curses flowing out from the gate to hell. Gilgamesh specifically mentions 5.6 billion curses for an unknown reason. It is a reaper that will strike down all humans equally, an endless darkness that is born from humans and massacres them.
Ichor leaks from the Grail when it is first being established. Its nature is a curse so strong that it is visible, so it can be called magical energy specialized for destroying people. It can not be practically used in any way, as it cannot be reshaped or anything of the sort. It stains anybody it touches with a strong curse and melts them as if they were being digested. The pain and fear felt before death remains as magical energy and becomes the next curse to seek out living people. It will kill anyone it touches unless they can remove it from their body before they are melted away. Those who are not dissolved will be assaulted mentally by "All the World's Evil", eventually going insane or left cursed by its after effects, such as Kiritsugu Emiya, who lives for a number of years while suffering under the curse until it causes his death.
If fully submerged in the curse, it paints space itself around the person, surrounding them in darkness and imprinting hell upon their brain by showing them unseeable darkness, ugliness that can't be acknowledged, and all the crimes of the people in the world that make them want to run away from it all. Those trapped with the darkness will eat themselves to death from pain and hatred, and they will be swallowed by it like the painted space. Avoiding such a curse and the very concept of blocking it are nearly impossible. Escaping should be impossible, so Kirei Kotomine is shocked that Shirou manages to do just that through only willpower.
Servants affected by the mud can be blackened.
Grail mud has also been shown to be able to block the view of Gilgamesh's The Omniscient Omnipotent Star, a clairvoyance with a "sense of 'sight' several levels above the ordinary". The Noble Phantasm was said to be capable of not only discovering hidden truths with a glance, such as the True Names of his opponents and Noble Phantasms, but also visualizing the possibilities of various parallel worlds if he so choose.
Tiamat's Chaos Tide has similar properties to the cursed mud, but is geared towards remaking life on the planet rather than simply destroying it.