the holy grail
noun [ S ] uk/ˌhəʊ.li ˈɡreɪl/ us/ˌhoʊ.li ˈɡreɪl/
(also the Holy Grail)圣杯（传说耶稣在最后的晚餐上使用过）
a cup believed to have been used by Jesus Christ at the meal before his death
极难找到（或得到）之物
something that is extremely difficult to find or get
Sustained nuclear fusion is the holy grail of the power industry. 能持久反应的核聚变是电力工业梦寐以求的“圣杯”。