The Holy Grail -- actually, there are two of them -- is an alien artifact of immense power associated with the Ancient God Katallakh. The grails are part of a two-key machine that controls the enthalpic state of the universe. When both are inserted into the machine found in the Tomb of Katallakh on the Moon, the universe can be destroyed or time's arrow reversed. Kids, don't try this at home, because it's potent stuff.

The grail has various other magical side effects based largely on its rapport with its possessor. On Earth, one of its RC2: The Claw of Templaro famous but trivial powers is to turn people into cow-like bovinoids.