"Its appearance, its shape, how it was contained... not a single record has been found." -Tama Minai
This anomalous mushi came into existence during a great disaster and sought to kill all forms of life. At one point, a Mushi Master sealed it inside a member of the Karibusa family. To this day, it continues to be contained by and passed down through the Karibusa bloodline, and henceforth those cursed with the black mark must write the stories of the mushi’s defeat at the hands of Mushi masters as a way of sealing them inside scrolls.
Though it currently takes on the appearance of a black, ink-like stain on the body part of its host, its original appearance is unknown. Very little is known about this mushi, and its very existence is kept as a closely guarded secret. It thrived during a great disaster which devastated plants, animals, and other mushi, and presumably poses a great threat to all living things.
No record of this mushi's appearance has been recorded, nor how it was sealed into the Karibusa family. All that's known is that Tama's ancestor sealed away the mushi into Karibusa family, and that the person whom the mushi was sealed into was pregnant at that time. As a side-effect of the seal, she developed coal-like marks on her body. Ever since then, a child has been born once every few generations with a similar coal-like birthmark. The body part which bears the mark is rendered useless.
The person with the birthmark is called a Scribe and they possess the potential to completely cure themselves by writing down stories of how other mushi have been killed. This process seals the forbidden mushi away in the form of ink on the scrolls, but the process itself is very painful to the scribe. The scrolls are stored away in the Archives, and Mushi Masters are allowed to view them in exchange for telling their stories to the Scribe. As the scrolls grow old, the paper decays and the ink must be copied into new ones. Till date, no one in the Karibusa family has managed to completely seal away the forbidden mushi inside them.
This mushi is known to be weak to stories of mushi being vanquished, but only if the stories are written down by the Scribe themselves. Simply hearing the story does not seem to be effective.