When I hear "holy grail," one of the things that pops into my mind is Jay-Z's song "Holy Grail." I didn't actually know this, but his whole album, released in 2013, was called "Magna Carta Holy Grail." These are obviously two very important terms in history, so I looked up why he decided to name his album this. This is the consensus that the internet seems to come to:
The whole point of the Magna Carta was to usher power back to the people of England and away from King John in 1215. It was kind of a revolutionary document that changed the whole meaning of the monarch. Jay-Z thought that this album of his would completely revise the way of hip-hop/rap. When he referenced the holy grail, he apparently did not mean for it to encompass any religious meaning. Rather, the word "grail" literally means "an object or goal that is sought after for its great significance." Jay-Z's goal was to successfully impact the music through his album.
I just thought this was pretty cool to be able to relate the actual history to such a modern concept.