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 Encyclopedia
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Kayneth’s mystic code made of mercury. It can shift shapes to suit any purpose; attacking, defending, scouting, etc. The mercury is filled with magical energy and is preprogrammed with multiple shapes. It automatically changes shape to optimally handle the situation, but this can also be a weakness if the enemy identifies a pattern in its function. The more complicated the shape it takes, the more magical energy is required to sustain it.

Translator’s Notes
  1. ^ Marrow of the Moon Spirit.
Fate/Zero Animation Guide I: Glossary of the holy grail war I
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Marrow of the Moon Spirit.
→See the entry on Trimmau.

Fate/Apocrypha material: Fate/Apocrypha Encyclopedia
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Supreme Mystic Code of the El-Melloi family.
The term Supreme Mystic Code refers to a Mystic Code sufficient to be called symbolic of a family of similar level to one of the Twelve Lords of the Clock Tower. In Volumen Hydrargyrum’s case, it was created by Lord Kayneth in his early twenties, a clear demonstration of the once-Lord’s exceptional talent.
Ever changing in form, it is a Mystic Code capable of performing both offensively and defensively without showing any openings. Its true nature however is that of a remote processor.
Upon inheriting it, Reines followed the advice of El-Melloi II and created a familiar out of it, endowing it with an elementary kind of artificial intelligence. It was the best advice for Reines who, rather than having the exceptional Magic Circuits of her predecessor, instead boasted a talent for precise control.
By forcing it to remember Reines’ precise control, the artificial intelligence is able to imitate it perfectly. The newly christened Trimmau being capable of taking so many various forms is primarily a result of this advice she received.
In addition, because of Trimmau’s nature, it has also learned a great many things from Flat’s magical hacking. Though that has included a large amount of unnecessary wit and famous sayings, Reines has thus far turned a blind eye to it for the sake of the other merits it brings.

Translator’s Notes
  1. ^ Moon Spirit’s Spinal Fluid.
Lord El-Melloi II Case Files material: Lord El-Melloi II Case Files Glossary