the Holy Grail

noun
1
the cup or bowl believed to have been used by Jesus Christ before he died, that became a holy thing that people wanted to find
جام مقدس
2
a thing that you try very hard to find or achieve, but never will
هدف دشوار و دست‌نیافتنی, هدف والای دست نیافتنی, آرمان دور از دسترس