The Deep Aqua Mirror was Sailor Neptune's Talisman. It was used to not only to reveal the truth, but also as a weapon. She could use it to perform the attack Submarine Reflection.
In the manga like with the other talismans the items were already possessed by their owners and rather than being made to summon the Grail they were made to summon Sailor Saturn. The mirror has been seen to work for others beside Sailor Neptune, Rini once used it to locate and teleport to a different location. Michelle gave Rini the mirror at some point before the Dream Arc. When the Outer Scouts returned, Rini teleported the mirror back to Sailor Neptune in order to help her fight the Dead Moon Circus.
In the anime, the Mirror was found sealed within her Pure Heart. And once it was brought out with the other two talismans, Space Sword and the Garnet Orb, the Holy Grail was produced.