For this technique a living sacrifice is required for the Black Cocoon to use as a vessel. The process which a human body is converted into an vessel is similar to the process of petrification that exists in nature, with certain plants. When a tree or plant becomes petrified, what happens is that over the course of years, the original biological elements of its cells are very gradually replaced by mineral elements so gradually that the infusing minerals take on the same shape and appearance as the original biological cells. In effect, the original biological cells act as a matrix, or as a sort of "molding", for the incoming mineral deposits and what you end up with is a gradual build up of minerals in the exact shape of a tree trunk, or a plant of some sort. This same process is what happens the seed of the black cocoon is planted within the victim’s body causing them to lose their will and become puppets of the wielder. The energy of the Black Thread flows into it and replaces all the original human organic matter but it does it gradually, and in such a way as to mimic the original cell structure. And the organic matter is not completely replaced, but is only 99.9999 percent replaced. A tiny amount of the original human cellular matter remains as a template, spread molecularly thin throughout the newly generated, unnatural tissue which is made up of necromantic energy in more or less solid form. The original soul attached to that body loses its adhesion and becomes knitted into the cacoon as it grows to eventually become a Black String Heart. The user can theoretically reincarnate a limitless number of people in this way, so long as they have enough sacrifices, and energy to perform the technique.[2]
There are two varitions of this technique in the first While they are subject to the Ura-Fuuchouin will bearer does still have free will. The second is when the technique is combined with the Cursed Weave and the Black String Heart is prepared before hand. Victims of this version are nothing more than puppets and while this does provide them some degree of free will they are little more then mindless killing machines under the Ura-Fuuchouin command.