Volumen Hydrargyrum

One of the mystic codes employed by Lord El-Melloi. Essentially, it is a mass of magically-infused mercury that has been filled with a large variety of movement patterns and programmed to respond appropriately depending on the situation. I guess you could call it a kind of battle golem? Out of all of Kayneth's armaments, it has the greatest capacity for physical destruction. He had prepared many other mystic codes designed for battles with curses and conjurations, but the vast majority of them were lost when his hotel was blown up by Kiritsugu.
Though it wouldn't be wrong to call it an almighty weapon due to its versatility, in the end it is still just an automaton, and once you see through its movement patterns it's fairly easy to handle. Also, since the magical energy required to operate it is directly proportional to the complexity of its shape, it is necessary to keep it in its basic form as much as possible—switching to a more complex form drastically reduces its reaction speed and power. The final “sea urchin” transformation used in the battle against Kiritsugu was a form that combined both raw defensive ability and cunning, but accordingly the drain on Kayneth's magical energy was immense.
By the way, after the war ended Lord El-Melloi II may or may not have reprogrammed this mystic code and employed it as a multi-purpose maid golem. Though it has enough intellectual capacity to perform simple household chores, it seems there's a troublesome bug that pops up every now and then that causes it to rampage around insisting that it's a killer robot from the future. Maybe it was shown a detrimental movie just when it was on the verge of completing its education in good taste…?

Fate/Zero material: Fate/Zero Encylopedia
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Marrow of the Moon Spirit.
→See the entry on Trimmau.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia