Mash Kyrielight's and by extension Galahad's large black shield. A shield made from the Round Table of Camelot where the Knights of the Round Table would sit. It is capable of repelling attacks, so long as the user's willpower doesn't falter. It also grants its user and Master an anti-poison ward, and is the catalyst used for summoning Servants in Fate/Grand Order. In the Olympus Lostbelt, it gains an additional cannon-like weapon called the Black Barrel, which is capable of destroying gods.
It was originally designed to be Tachie's shield, but her character was scrapped from Fate/Stay Night and repurposed as Mash for FGO.