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 such extraordinary amount of degree in both effort and natural talent, especially those who are unaffordable for extreme intensive combat situation, for example general commoners have more difficult time on activating and developing their own Aura power compare to combat-trained and combat-experienced warriors who are combat-talented, 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 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 "giving 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-giving" 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-giving" ritual: providing them the certain potion through drinking in order to "gain their own form of Grimm they transformed". After went through this ritual and turn back to their own tribes, they started to build their own influence
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 certain level, 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, 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 (they are usually not able to change into Grimm-form until they are around ten years old, despite still have 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. Both (initial and terminal) stage will not be fade away/revert back to human form until through either voluntarily mean or being knock out unconsciously.
[Society and Culture aspects]
Western (Sanus) tribes :
Eastern (Anima) tribes :