Spindle Creation

A grisly process which converts a human into a spindle automaton.   The spindle creation method is essentially a way of imperfectly forcing a darkness alignment, a la Cotd. Because of its usage of a fully developed human and the relative abruptness of the process (~3 days for a spindle vs ~5 years for a Cotd, plus previous generations of Cotd), a lot of physical characteristics and abilities are sacrificed. The original ritual was based on fragments of information from The Tablet, passed on through word of mouth.   The ritual itself technically only encompasses the portion where a puppeteer is forced onto the mundane plane and fused into the body (the final two steps). This is the only real black box aspect of the process- the rest of the ritual has been adapted and improved in many ways throughout the ages. For example, various grafts can be added to the body before this final fusion step in order to change the resulting spindle; one could replace certain bones with orichalcum for a much higher magical energy capacity and usage efficiency around the given limbs. This would consequently require advancements in medical technology to pull off these grafts etc.      

Mechanism & Development

Spindles are a combination of several dynamics.   They're a first-order soul with its MSI (Mind-Soul Interface) systems replaced by a bound puppeteer. At the same time, they also lack a mundane body, instead relying entirely on puppeteer possession.   Efficacy of possession depends heavily on material properties. Substances with higher magical capacitance are overall better, as there's plenty of time to soak. Claim strength stability is a tradeoff: the higher it is, the more can be utilized, but the less stable the result.   Echo posession means that by default there's a total pool of energy to use for movement. This energy can be converted into force spent to reshape their form, and the efficiency of this conversion depends on the material. It also hits diminishing returns- the more energy input, the less force output. This makes materials such as living clay extremely important, as they're specifically malleable to magical force. Larger quantities of clay provide more material to easily act upon for higher maximum output. However, this comes at the cost of overall efficiency, since it's more mass to move, and in many cases, more mass to constantly offset in order to remain balanced. Additionally, energy has to be spent simply to maintain claim on material, so more mass adds additional upkeep cost. This is mostly proportional to magical specific heat.   Because the spindle's body is basically a puppet being moved by an external force, certain principals of leverage are ignored, leading to a few unintuitive consequences. A spindle can literally pull themselves up by their boostraps because the force technically doesn't come from within them. At the same time, they generally move using patterns similar to human ones due to the other side of their existence, intrinsically being human souls with (generally heavily suppressed) human instincts. While the theoretical efficiency of levitating may be better under some conditions, the added inefficiency layer of translating this unintuitive use of energy through the MSI means that it generally comes out as worse.  

Material Suitability

  There are essentially four major material components to a spindle:  

Spiritual

Directly linked to the spindle creation ritual, which generally creates some sort of spiritual core with a physical manifestation shrouded in shadow; and hard light components.  

Articulation

Articulation: used at joints, directly acted upon by the spirit's energy. Most commonly living clay. Most important is magical conductivity and malleability through magical means.   Living clay is a very close to optimal answer to articulation points, the truly superior options requiring extensive alchemy not reached for centuries (and even then, being significantly more expensive).   Stormglass also technically works as articulation, and is used as such in some very late-era alchemic works. The primary issue is working it in the first place: the type of magical energy concentration to make stormglass malleable also generally tends to disrupt the spindle binding ritual. A combination of binding-defined thresholds and alchemical manipulation techniques developed around 750AM managed to pull this off effectively. Stormglass is far more responsive to magical force, but also requires cladding to surround it, as it will act as a liquid to external forces under the types of magical energy concentrations involved. It mostly comes up in heavy spindle units with dedicated technicians who can compensate for the added instability. Additionally, stormglass articulation encased in mundane glass became a fashion among the nobility at some point, as it provided a rather interesting aesthetic.  

Structural

Structural: used to connect joints, for limbs and such.   Strength to weight is very important, as the material isn't directly acted upon. Magical conductivity and specific heat are also important to dictate how efficiently the spindle can possess these components. Energy travels along structural components starting from the spiritual core, so all articulation must be connected by structural material. The efficiency of energy transfer is dictated by structural material in a way much like electric resistance (higher thickness = less resistance etc.).   While appearing to be analogous to bones, structural components are closer to a mix of nerves and blood vessels in terms of function: they carry the instructions and energy used for motion, not actually being very good at maintaining structural integrity. The origins of the term 'structural' link back to the fact that it's these components that define the layout of the construct, generally being bound to humanoid shapes (due to being built with human sacrifice). Deviating from this is possible, especially with more advanced alchemy developed in later years, but it's more common to instead graft in other structural materials according to the existing structure. As the construct is driven by a human mind, it's easier to match the body accordingly.   Deviating from the bound mess of flesh and bone found in standard structure is the purview of more advanced alchemy. There are numerous ways to graft differing materials, generally involving some level of fleshcraft before actually putting the subject through the ritual.   Structural components will form natural umbrite substructures to slightly reinforce them, mostly increasing toughness without affecting stiffness. When broken, they appear to be infused with an inky black substance- signs of the puppeteer's posession.   Orichalcum is a material that brings several tradeoffs: it's far stronger, but also significantly heavier. While the strength to weight and magical conductivity is better, it also results in a far higher upkeep cost due to weighing more. The net weight can be offset by a reduced need for cladding, as the orichalcum will naturally reinforce itself through the claim, falling into a durability generally around that of mild steel. The energy requirement to properly possess orichalcum can easily fall within the bounds of inducing rampancy in the spindle, especially if larger portions of the skeletal structure are replaced. Most orichalcum spindles only have a limb or two replaced- even that is a dangerous proposition due to the potential variability in rampancy thresholds. To create an orichalcum spindle is considered a rather exceptional feat of alchemical prowess as a result. Also note that the ritual requires the subject to be alive as it works- surgical skill is consequently rather important in all this. It also makes it impractical to replace certain parts which would theoretically result in a much better end result, such as the spine.   Other noble metal infusions, not being intrinsically linked to Blood (from which the spindle creation ritual draws upon to act as an interface), can disrupt the possession process. Moonsilver especially can completely destroy the interface between puppeteer and human soul, often destructively ending the ritual should it be attempted on a moonsilver-grafted body (note that this also applies to Cotd).  

Cladding/Armor

Cladding: used as a protective outer layer.   The primary difference between structural and cladding is that the latter isn't part of the entity's claim. Not being part of the claim means reduced upkeep cost, and allows one to discard considerations on magical properties. However, structural components must connect articulation in order to allow energy to flow to them. Cladding actually tends to be more important to protect structural components than articulated ones, as the latter exhibit a level of self-repair abilities. Structural components tend to focus more on being lightweight and durable than their receptiveness to magical manipulation.   Along with protection from the elements, cladding is often used for insulative purposes. Energy naturally leaks from the possessed body- ambient energy diffusion. As a result, cladding bodies in insulative materials reduces this loss. It's harder to do with regard to living clay, as the material must flex, but most rigid surfaces can be painted over with an insulative substance such as binder's resin to dramatically reduce upkeep cost.   Materials suited for physical cladding are similar to those worn by humans. Metals for more protection at the cost of fatigue, and varying types of cloth for general insulation and protection from abrasions.   Important to note is the role of insulative cladding in maintaining spindle subservience: while cladding does reduce energy loss, it's actually much more important to prevent energy gain- blocking the spindle's ability to drain ambient life. An unclad spindle is only really somewhat more inefficient than a clad one in terms of energy retention, but is capable of consuming far more- making it far easier to tip into rampancy. An army of fully clad spindles on march could cause the fields they march through to dull and slightly wither; a platoon of unclad spindles would completely decay the grass that they walk over (before going rampant and ignoring commands). This plays a large part in the difference between Eideli spindles, and those created by the Patchwork Kingdom-