Beginning as smaller tribes among primitive tribal world in earliest age of the Second world of Remnant, in order to find better way for combating their Aura-empowered warlike opponents who raid their settlement continently,
Footnote: Although Aura power could be used by anyone 'with a soul' in theory, activating and further developing such power could quite be almost difficult for even ordinary people to withstand without putting in and undergoing such extraordinary amount of degree in terms of both effort and similar-to-near-death experience and lastly, which is most essentially, involving such same above-mentioned amount of degree of inborn prerequisites, including both chances and natural talent, especially for those who are unaffordable for involving extreme intensive combat situation, for example ordinary commoners have more difficult time and fewer chance on activating and developing their own Aura power compare to combat-trained and combat-experienced warriors who are combat-talented, which mean that it's quite problematic for smaller tribes who have not enough populational amount to provide considerable supply of warriors to defend from Aura-empowered raiding opponents. And even the one using Aura-activation drug/herb/potion, not only need to be made of considerable high-qualitied ingredients and conducted relative-highly-complexed ritual, but also depend on subsequently additional extensive training to keep Aura's quality improved and consistent, otherwise Aura-activation and its quality of consistency isn't 100% guaranteed, including having weaker protective force field and power fade away after relatively short time.)Several important figures from their own tribes, including tribal leaders, elders and religious figures, including shamans, had reached their common consensus after certain inter-tribal gathering event, which meant "granting tribal members the ability to transform into some forms which looked like Creatures-of-Grimm in order to have better chance to fend off Aura-empowered raider who invaded their living place and took whatever their had" in short.
In order to carry out their "superpower-granting" plan, besides preparing associated ingredients of Grimm-form shifting potion and further refining potion formulation, they not only contacted with many smaller tribes, who not only from eastern-Sanus, but also some from far-away Anima as well, and also held certain massive gathering event for inviting participated tribal representees (usually as either tribal leaders, elders, shamans or well-known warrior) in order to practice "superpower-granting" ritual: providing them the certain potion through drinking in order to "grant their own form of Grimm they transformed" (along with other "precautional measures" in case of "incident prevention". After went through this ritual and turn back to their own tribes, they started to build and spread out their own influence range for the aim of making their own tribes more safer, despite facing other adversaries including [Spirit-folk] and [Silver-eyes groups]. Among all of Grimm-folk tribes, the [Oni demon] tribe had most significant range of expansion in terms of influence, and even to certain level launch the large-scale widespread aggressive raiding wave against numerical amounts of settlements, especially in eastern-Sanus, and caused serious damage on those settlements which is historically called as "Dark-tides", not just these settlements like [Spirit-folk] and [Silver-eyes groups] on eastern-Sanus were affected, but also other non-[Oni demon] tribes as well, as the raiders of [Oni demon] tribes would view anyone who was not [Oni demon] tribe as the target of raiding. Nevertheless, there were some [Oni demon] tribal individuals who chose a more diplomatic method which demanded non-Grimm-folk settlements give a few of their female villagers as marriage mates for their tribes to increase their tribal population as the exchange for either the discontinuation of raiding said settlements and leaving these settlements alone or settling a 'protection agreement' with said settlements and ensure to keep these said settlements the certain distance for their tribes and no longer disturbance and harassment on those said settlements, instead of just raiding these settlements. And the massive infighting among multiple factions within the [Oni demon] tribe finally brought an end to this series of raiding events.
Footnote: despite the persuasion from certain faction led by so-called {dark woman} for demanding the Grimm-folk tribes to "join her side", the Grimm-folk tribes' strong and fierce attitude on their own independence and autonomy had made their having a strong desire against under so-called {dark woman} and her associated faction's rule, combining with the "external unknown reason", eventually so-called {dark woman} and her associated faction gave up on the idea of putting the Grimm-folk tribes under their rule.)Following much more widespread phenomenon on the application of Aura utilization and rising amount of Huntsman-like fighters (along with long-term holdout of certain groups like [Silver-eyes groups] and 'other factors'), overall population number of all Grimm-folk tribes have been generally largely decreased than before, especially for [Oni demon] tribe having more significant populational declining due to both external force event (e.g. fighting against groups like [Silver-eyes groups] during the "Dark-tides"] and internal conflict (e.g. civil war between their own tribes)). Even though there were several reported appearances of Grimm-folk's involvement during the Great War, such accounts were rare enough to considered as baseless rumors. Current, most of their population are mainly living in much more hidden and remote places like mountains, islands and underground caves, where are usually far away the Remnant's civilization area such as [the Kingdom of Vale].
When changing into Grimm-form, their sclerae will change from white into black (from iris peripheral spread to sclerae peripheral) at first while their iris will glow at a certain level (which is usually used for showing the aggressive emotion like being raged, intimidating the opponent or served as warning sign of either "ready for any violent action" or "preparing for the battle/fight"), and then the black linear strips, which are unique according to their own Grimm-type, will grow on their own body, covering and spreading from face to the end of the limbs, then their own series of specific Grimm-like matter will be produced from black linear strips on the external layer part of their body (or between epidermis and dermis), and finally transform into their own specific Grimm-form along with usage of their own Grimm-form's associated abilities, such as transforming into certain-giant-Grimm-monster-like form and producing Grimm-like matter which provide additional functionality (they are usually not able to change into Grimm-form until they are around ten years old, despite still having some exceptions, and they consider this event as a rite of passage).
When in Grimm-form, even in initial stage of Grimm-form transformation (in short, still staying in human-look appearance while without utilizing any Grimm-form's associated abilities), this form can negate Grimm's sense and mask their "human/soul presence", making them seem "invisible" and "undetectable" to the Grimm which able to reduce the Grimm's attraction and even further decrease the Grimm's aggression and hostility tension toward the tribal members. If any parts (like cut-off remaining parts)of Grimm-like matters which the Grimm-folk produced are remained, these parts will be self-evaporated after a while, and Grimm-form in both (initial and terminal) stage will not be faded away/reverted back to human form until through either voluntarily mean, being incredibly fatigued or being knock out unconsciously. And furthermore, overusing said ability under abnormal circumstances including under serious fatigue could lead to great amount of muscle mass amount reduction and in more severe case would cause potentially fatal consequence.
In terms of Grimm-form transformation, it could be cataloged into [partial form] and [completed form]. Partial form is usually applied on between the initial stage and terminal stage in Grimm-form transformation and only have access of half the amount of Grimm-form abilities, for example only one giant arm part is grown out from [Giant wolf]'user's single arm or only one additional arm part is grown out from [Six-arms demon]'user's body. Meanwhile, completed form is usually applied on terminal stage in Grimm-form transformation and have full access of whole amount of Grimm-form abilities, for example [Giant wolf]'user is able to transform into giant-wolf Grimm-form completely or all of four additional arm parts are grown out from [Six-arms demon]'user's body. Comparing to the latter one, partial form has lesser amount of usage risk, more energy-efficient, more operationally refined and more utilitarian, despite having only half an amount of strength and power of completed form, and also relatively harder to achieve especially in the first attempt.
In terms of regenerative healing ability, although all types of Grimm-folk have the regenerative healing ability in terms of certain theoretical aspect, not all types of Grimm-folk have the strong regenerative healing ability at the same level, for low-level ones are either heal the wound slowly or still need additional care in order to accelerate the healing process
Despite having similar appearance compare to the true Grimm such as [Giant wolf] tribe to Beowolf, there are some differences between them, for example the color of Grimm-form's main body (Giant-Grimm-monster forms especially) which the Grimm-folk constructed is mainly purple rather than pure black, harden armored parts (like bone armors) are blackish purple instead of white, eyes-color are accorded to said user's own eye ones instead of just red or yellow.
Due to utilization of Grimm-like matter which typically block (sometimes descripted as "being absorbed limitedly") and even cut their access to use Aura, they can't use Aura at the same time when they are in Grimm-form, though the utilization of the Grimm-like matter (and further, Grimm-form) the Grimm-folk tribes used itself don't neither affect nor cause drainage and depletion of Aura energy of Grimm-folk tribe members, as both of the Grimm-like matter the Grimm-folk tribes utilized and Aura are separated power systems. In terms of comparison between the Grimm-like matter which the Grimm-folk tribes utilized and Aura, despite lacking protective energy barrier the Aura provided, the Grimm-like matter can be used almost instantly without the need of preparation for being activated at first, have much better regenerative healing performance than Aura, don't have the concern on being depleted when using the Grimm-like matter, which the Aura do, and lastly don't have the risk of leaving themselves to be vulnerable or undefended while recovering the energy barrier (in a certain timeframe) after being broken which the Aura needed. Nevertheless, there are some conditional situations that Grimm-folk could have access to using Aura power (*being viewed as being theoretical by most of the Grimm-folk): either using Aura power while not in Grimm-form or [in more recent case] summoning Auric-spirit from themselves with the cost of cutting off access to Grimm-form transformation ability. In terms of practice of Aura usage, said practices are generally unutilized among Grimm-folk tribes in eastern-Sanus, still said practices in certain forms are applied by some Grimm-folk tribal members in Anima "for additional health benefits".
In terms of reproduction, when both of two parents are same type of Grimm-folk (such as both of two parents are [Oni demon] tribal members), the children of such unions typically always have same Grimm-form type which is passed from their parents. When each of two parents are different types of Grimm-folk (such as one of two parents is [Oni demon] tribal member while the other one is [Giant wolf] tribal member), the children of such unions are typically always inherited with one of two Grimm-form types from their parents (for example the children of [Oni demon] parent and [Giant wolf] parent will be received either [Oni demon] Grimm-form or [Giant wolf] Grimm-form) (except for the case of [Perfume succubus]: whether the union's counterpart is Grimm-folk, human or Faunus, the children of such union are always be [Perfume succubus] (along with inherited birthed ratio trait of having about 85 to 90 percentage chance to be birthed as female)), and also do not appear as hybrid infants having mixed traits of both Grimm-form types. In terms of the union with either human or Faunus, despite being genetically comparable between both sides, the children of such union are always Grimm-folk (of any type) at all, neither having other birthed human or Faunus siblings along with the Grimm-folk infants, nor appearing as hybrid infants having human or Faunus' traits mixed in (,which in short do not have any additional animal traits such as having an additional pair of cat ears). Lastly, in terms of the union with [Silver-eyes groups], their birthed descendants are typically always either very short-lived or have severe genetic defective illnesses, which typically result in death after birth in a very short time in both cases.
[Society and Culture aspects]
Among the tribal world of the Grimm-folk, there are several overall differences between western (eastern-Sanus) tribes and eastern (Anima) tribes.
Most of Grimm-folk live in large-scale tribal society under the hunting-gathering lifestyle.
smaller close-knit clan and community groups
wandering core-family
"abyss-people"