The Holy Grail is an Event Item found in Stage 4 of Riviera: The Promised Land. The Golden Key is needed to obtain it.
Acquirement: It is located in St. Claremonts in Tetyth within the room marked "H" below. The Golden Key is needed to open the door (marked D) in the room that leads to its chest.
[D]-[H]-[ ]-[ ]
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Function: With fourteen (14) Battle Items or less in the inventory, a Holy Water x5 can be had at Undine Spring.