During the quest, King Arthur informs you that it has just passed between dimensions into RuneScape. Should you choose to accept the quest and retrieve the Holy grail, you must travel to the Fisher Realm. If you try to take it before it's time, you will receive a message saying that you're not worthy of it. Its only use is to finish the quest by speaking to King Arthur while it is in your possession. Having one holy grail in the inventory and trying to take another gives the message: You feel that taking more than one Holy Grail might be greedy...