Black Mud

In the Third Holy Grail War, the Holy Grail took in All the Evil in the World (Angra Mainyu) and was tainted. The black mud is what is pulsating inside the tainted Holy Grail. The mud’s nature is to deny each and every form of life. After being tainted the Holy Grail still fully maintained its function as a wish granting machine, however the manner by which it grants wishes is now twisted by evil beyond imagination. Normally anyone that fully bathes in the mud is decomposed and absorbed, but Gilgamesh is no normal Heroic Spirit, and when he bathes in the mud it is unable to consume him and ejects him as a foreign object. As a result of this Gilgamesh reincarnates. Kiritsugu is affected by the evil powers of the mud, which eats through his body destroying 80% of his magic circuits.

Fate/Zero Animation Guide II: Glossary of the holy grail war II