After a marathon session of Fate/stay night (all three routes, with inadequate sleep between routes, and too much caffeine and sugar) L33t creates something. A tiny thing, so small that individual molecules are important components. A self-replicating nanorobot. Then he crashes, hard. When he wakes up, he finds that the original nanobot has exploded (a
very miniscule explosion, far weaker than a bee's sneeze), but the other nanobots it built have mostly not, nor have the ones they built, nor the ones that group built...
Of course, just self-replication wouldn't be scary enough, especially when they tend to break down (and get cannibalized for parts) eight to twenty-four hours after being made (which is only one of the reasons a grey goo scenario is not an option; another is that while they reproduce fast enough to expand their numbers, the speed necessary to go grey goo is
vastly greater than they are capable of). When infecting a living human body (or under special circumstances, other bodies), they build partially-extradimensional structures 'in' the nervous and circulatory systems that act very much like the Nasuverse's
magic circuits. There's more to it of course, but if you don't care about that, you could skip down to the dotted line under the TL/DR summary.
How many circuits any one magus has, and the capacity of each circuit, seems to be determined by a number of factors, environmental, biological, and psychological. The main advantage of a large number of circuits over a high capacity-per-circuit is the ability to power more than one magical operation (like a spell, or supplying energy to a
Mystic Code or
Familiar) at a time - actually
casting more than one spell at a time would require quite good multitasking. Either way, the fewer circuits used for an operation, the less power is available for it.
The Virtual Intelligence (in this context, an artificial intelligence that is not truly sapient) of the magic circuits determines the subject's
Element(s) and
Origin - and in some cases, a
Sorcery Trait or
Mystic Eyes, or similar - based on existing traits (and if already triggered, based on their powers and shard). For example, if Taylor gains the nanites after she triggers, her Element will probably be something like 'Worm' and her Origin will be 'Administration.' Someone who triggers despite having magic circuits may change Origins, or have dual Origins, and may gain another Element, if the Element they start out with does not fit their powers well enough. For example, if Dinah's magic circuits give her something like Earth or Fire as an Element, and she still triggers with her canon power (which is reasonable, since the sort of stresses likely to produce that power probably wouldn't be reduced by those Elements), she might add Ether or Divination to her existing Element or Elements, and her Origin could change to something like Probability or Prediction.
The magic circuits' VI will also work with the new magus's subconscious mind to develop a system of
magecraft compatible with the user's personality, skills & interests, other mundane traits, and their Element(s) and Origin(s) (and Sorcery Trait(s), if any). The new magus can become aware of the system as if they were developing it themselves, though they will gain it much faster than they would without help. However, they can also fail to become consciously aware of their power, especially if they are lacking in intelligence or imagination, or have some personal, cultural, or philosophical reason to reject their powers.
Generally, a new magus becomes consciously aware of their abilities from 'opening' their circuits in a moment of stress, like a much milder trigger event (though it could happen in concert with an actual trigger event). After that point, 'opening' and 'closing' the circuits becomes a reflex.
The construction of Mystic Codes is in this variant closely related to the making of Familiars, to the point that one could say that a Familiar is a type of Mystic Code, or vice versa. The magus's blood, hair, or other bodily material is incorporated into the item in a ritual appropriate to the item's purpose and the magus's magecraft style, which gives the item magic circuits, though circuits that mostly cannot fuel themselves, instead needing external power, usually from a magus (this does not reduce the number of magic circuits the magus has, instead granting lower-quality replicas); this is easier if the material was created by your power (or modified, or otherwise related to it), or significant to your Element or magecraft style. If the item is to be a Familiar, an extra step is required, in the form of a ritual to 'collect the leftover thoughts of the dead' (create a VI based on some person or animal that died not too long ago, using a retrocognition effect to gain the necessary information). Creating a 'spirit' (incorporeal or semi-corporeal projection) as a Familiar is also possible, though this means that the magic circuits are almost fully extradimensional, and the spirit's ability to interact with the physical world will be somewhat limited. It is theoretically possible to 'steal' a parahuman's projection by doing this, but it is unlikely that the projection will cooperate under most circumstances.
It is possible to turn a living creature into a Familiar, skipping the otherwise-necessary step of creating a VI. Such a creature would have magic circuits that recharge on their own, but would still be of lower quality than the master's circuits, and would likely have a narrower Element or set of narrower Elements. Also, doing this is instinctively viewed as rather creepy by the vast majority of magi. The exceptions to this would tend to include capes whose power involves controlling or enhancing a specific type of creature, or those for whom other factors overwhelm any sense of disgust they might have.
Because the magic circuits are partially extradimensional, reverse engineering them would be very difficult, even for Dragon Unchained.
TL/DR: L33t makes a thing. The PRT may want to kill him for this thing, because Nilbog. His shard is excited.
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Now, L33t isn't totally suicidal - much to his shard's disappointment, he's pretty cautious - but he can't quite bring himself to just destroy his creation. So, he uses an old teleportation device to send the nanobots to another city, at the teleporter's maximum range. He thinks there's about a 70% chance that the nanobots will be destroyed (in fact, it's closer to '70% of them will be destroyed, most of the rest arrive safely'), and even if they aren't, he's not the one who'll be blamed for it (he hopes), since the PRT should trace the first infection to Chicago (or New York, or London, or whereever, the point is that Patient 0 will
not be in Brockton Bay). Let us say that the infection is not discovered early enough to be contained (possibly because the nanobots arrive near an airport), and makes it back to Brockton Bay within a month - which may well be before the government knows about it.
I'm not entirely clear on when this should happen, beyond 'after Uber and L33t have been operating for a while,' and 'before canon start.' See, there's less than three months in canon between Taylor's first outing as a cape, and the end of the Warlord Arc, which is probably too short a time for her to notice she has circuits, develop a form of magecraft, and get good enough at it to be useful, if she's also going out and being an active cape in a story with really tight pacing. If she gains the circuits before or not too long after her trigger, she has time to add it to her preparations (and if it's before she triggers, she might not trigger at all, or might get something different from her shard).