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Summons are a branch of Alhazredian magic that has survived to the modern day, like the Familiar Uplift and the Time-Space Distortion Barrier. Bound Familiars and Uplifted Familiars are similar but distinct classes which will be discussed in separate sections.
While terrifyingly powerful, Alhazredian magic in general was not particularly fast or efficient to cast. Thus, the priest-kings of Alhazred developed other ways of making war. They built advanced magitech arsenals of autonomous weapons, and gene-locked them, storing them within the genetics and Linker Cores of every existent member of the bloodline they belonged to. Then they taught their mages the methods to unlock these arsenals and summon up the constructs within. These magitech-produced creatures were similar to the Wolkenritter, in that they weren't really living beings, but rather manifestations of Lost Logia programs - only in this case, the Lost Logia in question were entire lineages, and the programs were encoded in the Linker Cores of everyone in whom the genetics ran strongly enough. The summoning spell realises this potential, taking the dormant program and turning concept into fact, bringing it into the world. A summoner will access the arsenal and have a user profile created, which retains information and experience between summonings. Note, also, that without Alhazredian magitech to reinforce the programming, diluted genelines lead to loss of information. An "impure" summoner won't have the higher tiers of summons encoded on her Linker Core, and thus won't be able to summon them. A weak one will have the codes, but not the magic necessary to bring them into the world. To use the strongest of the True Summons of Alhazred, both purity and magical strength are required. Lesser summoners can cast in concert, pooling the different, damaged versions of the same code they have together in the hopes of, between them, reconstructing the whole thing. This means that the higher-level summons often need five, ten or even twenty summoners patching the holes in one another’s codes and pooling their power to summon the stronger creatures into being. The apex summons often require the purest and most powerful of a summoner clan to work together to call them to battle.
And how strong they are! Some of the constructs in there, as wars between the priest-kings escalated, became titanically powerful, requiring many centuries of careful labour, development and construction to bring to fruition. This arms race culminated in the likes of Voltaire and Hakutenou, weapons of mass destruction that could devastate entire regions trivially as they fought. Cults formed around these monsters - Caro is a priestess of Voltaire - and rightly so, for these are the pinnacle of True Summons, and only those lucky few with strong enough Alhazredian lineage of the right type can summon them. Caro is a fluke and a prodigy, recessive genes activating strongly enough in her that she has high-level access to the arsenal her tribe carries, and the destructive potential that access carries with it is a rather clearer explanation of why they shunned her - she is effectively a walking nuke without full control over the things she can summon. And worse yet, she can summon things singlehandedly, and thus keeping her away from other summoners when she is stressed is not enough to stop her from dropping a thirty metre long fire-breathing lizard on her tribe’s camp and then losing control over it, if she feels threatened, scared or upset. And in a nomadic clan living on a Type-3 world filled with enormous and often dangerous beasts, there are many things to be afraid of.
Notably, the existence of so-called True Summons actually is a legacy of a weakness in the Alhazredian way of doing things. Simply, compared to modern magical styles, their magic is crude, power-hungry, inefficient, and slow to cast. It is perhaps a sign of their culturally alien way of doing things that their solution to "most people cannot use magic well, certainly not in combat" was not to work on their style, but was to design magical weapons system that just require raw power from the caster to operate, and which do the hard-coded casting themselves. The fact that they had to design these things, rather than using modern-style Devices, shows a rather crude workaround for a fundamental flaw with their magic system that shows that just because something is older does not mean that it is better. From one perspective, while these summoning bloodlines are dedicated hardware, a modern device is a Turing Machine. From another perspective, the closest equivalent to the bloodlines, in the modern era, are the combat cyborgs, but they, like the Belkan Sankt-Kaisers, show a much more efficient and effective way of doing things, as they directly provide both power and functions for the user, while the summoners take power to obtain functions.