The advanced version of the “Volumen Hydrargyrum: Marrow of the Moon Spirit” Kayneth El-Melloi Archibald used for battles of magecraft in Fate/Zero. Currently, the legitimate El-Melloi successor, Reines, is using it as a mercury maid. She (the maid) specializes not only in acting as a bodyguard, but also in using her body to clean and invade places.
People think that she was made by Lord El-Melloi II, but he actually only gave advice to Reines. There are still many secrets regarding the full details of her abilities. These details should be brought to light in “Lord El-Melloi II's Case Files” (Author: Sanda Makoto), so please look forward to it. (It's my style to throw difficult subjects at Mr. Sanda.)

Fate/Apocrypha material: Fate/Apocrypha Encylopedia