I would say that "The Holy Grail" is the name (a trademark if you wish), like is "True Cross".

So, True Cross, upon which Jesus is believed to have been crucified or similarly, The Holy Grail, which is believed to have special powers.

We can even go further: a fragment of a cross we believe is a fragment of the True Cross, upon which Jesus is believed to have been crucified, a grail we believe is The Holy Grail, which is believed to have special powers.