Agrius Metamorphosis

The Boar of Divine Punishment. The pelt of the Calydonian Boar, a Monstrous Beast which the virgin Goddess Artemis let loose upon the earth. It looks like a wild boar, but that is merely because the lifeform that was used as its basis just happened to be a boar. Just as depicted in the novels, if the pelt is worn by a person, they will become a Monstrous Human rather than a Monstrous Beast.
It grants power equivalent to that of A rank Mad Enhancement and grants an A rank Transformation skill, bestowing various characteristics depending on the environment. In exchange, the wearer loses their reason and, depending on the situation, becomes unable to discern and identify even their Master. It is a Noble Phantasm whose use pretty much equals self-destruction, and its use has never been confirmed in any of the subspecies Holy Grail Wars.
However, even the author himself couldn't help having doubts about how it transformed into bird wings for Atalanta, fused with her bow and shot arrows weaved out of mana. Was it supposed to be like the T virus or something?1

Fate/Apocrypha material: Fate/Apocrypha Encylopedia
Translator's Notes
  1. ^ The last line is a Resident Evil reference.