There are two mulberry trees in the game: one at the manor of shonagon Fujiwara Yorichika, and one at the Silkworm Shrine where Utsuki lives. These trees were planted by the Hata clan, influential people at the time, so their properties are completely different from those of normal mulberry trees. Normal mulberry trees bear mulberry fruit, but those sent by the Hatas create white silkworm eggs.
The silkworms born from this "fruit" possess a strange power, wherein if they spin a cocoon around a body and a recently-deceased creature they can bring the person back to life. However, even if one is resurrected by the power of the silkworms they will not be resurrected completely, dying after a while. To prevent this, they must go back inside the cocoon. From the second time onwards, after creating the cocoon they cannot be revived by silkworms alone, needing to create a cocoon with another being and absorbing its "grudge" as nourishment. Futhermore, with each restoration they need more sustenance. At first even things like bugs are enough, but it proceeds with the second time requiring a small animal, the third requiring a human, until ultimately the absorption of someone with a deeper "grudge", such as a lover or relative, is needed.
If this revival is repeated nine times a "mulberry fruit" is born from the cocoon, from which sprouts a new mulberry tree. In other words, the mulberry trees use "fruits" in the guise of silkworms to create nourishment via grudges to absorb, trying to make its seeds flourish.
Those used in the rite may already be dead, but those who have already performed the rite themselves cannot be used. This is why the monsters attack the living.
Cocoon created by the "fruit" of the mulberry trees are extremely fragile, and cannot revive creatures if they are not properly looked after. If the cocoon breaks during the fusing process, in the early stages a mere mass of flesh, or even if almost complete a gaki, will be born from it. Also, if the the base creature is of poor quality, or if its compatibility with the creature being used to resurrect it from the second time onwards is bad, it will fail, so the cocoon must be protected and managed. The twin sisters are the ones who play this role.
The twin sisters are not humans, rather manifestations of the mulberry trees that appear in an attempt to create descendants of themselves. The reason they put the delicate cocoons into strong wicker chests and take care of them in order to protect them is so that the cocoon will safely create a new mulberry tree. The girls also search for material that can withstand nine resurrections. The twins appear before Utsuki and her sister, leading Utsuki to cause Kureha's fall, to turn Kureha into a "fruit". They probably wanted her thinking that as a daughter of Ashiya Douman, inheriting his powers as a diviner, she would make good material.
Course of the ritual
The "fruit" (silkworms) born from the mulberry trees have the ability to revive dead creatures by fusing with them, but the silk thread they spin each time they fuse with something also has special powers. Aside from being able to repel a variety of magical forces, cloth woven from this thread can also be used to create a barrier, and it is this thread that is used to create the barriers on doors in places like the manor. However, these barriers are not impenetrable. If the blood of a being that has been revived the same number of times as the silk thread used to make the barrier is put on it, the thread will melt. For example, fifth-time silk thread can be removed with the blood of someone who has entered the cocoon five times. From seeing Douryou used the "bloody cloth" on the seals in the manor, it appears as though Douman and the others have long been using this silk thread to perform their magic. Also, the magical power in the silk thread strengthens with each fusion.
The clothes Douman wears are also made of silk thread. Due to its strong repelling of magic, he evades most attacks.
Various symptoms show on the bodies of those who have performed the rite. As time passes, red granulation tissue appears here and there on their bodies, resembling mulberry fruit, their souls mix with that of the being with which they have fused and begins to cause mental abnormalities, etc. As you can see by looking at Kureha's diary, the symptoms become more severe the more time has passed since the fusion. When Utsuki and Kureha merge, sometimes Kureha's will mixes with Utsuki's thoughts, which is also an effect of the rite. Also, upon entering the cocoon they lose a definite image, sometimes taking on the outward appearance of the creature they fuse with. The Mizuhiki and Mukadebito (the lady of the manor) are exemplary of this, and in these cases their mentality strongly leans towards that of the thing they have absorbed, most of the original person's consciousnss lost.
It seems as though seeing these symptoms the people of the manor, who were unaware of the reality of the ritual, supposed those who had undergone it to be afflicted with a terrible illness. There are signs that for some time the Sick were being nursed at the manor, but this could be thought to have caused the progression of the people of the manor into monsters.
Ashiya Douman's goal is to defeat the strongest diviner in the capital, Abe Seimei, and reach the top of his profession. To do this he creates a manor beneath the Silkworm Shrine and uses it as a hatchery to research the silkworms, trying to obtain silk thread from higher-stage cocoons to increase his own magical powers.
Douman, who wants to obtain silk thread created by the Kuon rite, and the twins, who want more comrades created through it. Though they have different goals they share a common interest, cooperating and one by one using the manor's residents as bait in the ritual. In other words, the reason the manor is now the dwelling of monster is down to Douman and the Twins. However, no matter how far his research progressed Douman was unable to obtain a ninth-time silk thread. This is why there is no "nine" door seal. Incidentally, Douman wandered around sealing these doors, and when they were unsealed it would tell him how far his "test subjects" had progressed. Also, by combining the seals he also prevented the rite progressing any further without his knowledge.
Continuing his research, Douman one day learned that his own daughter Kureha had taken part in the Kuon ritual. Realising that she made a good "base", Douman lured his disciples to the manor in order to complete her ritual. This was also, of course, to nourish Kureha.
As Douman intended, Kureha absorbs one of his apprentices, Douryou, moving on a stage of the ritual. However, he never dreamt of himself becoming the last one to complete Kureha's Kuon ritual.
Many bodies hang in the Temple Room. Were these research materials too?
The puppets in the Doll Room were used by Douman in his research to test the magic of the silk thread.
The story of "Kuon" is set up in a complicated manner. It is difficult to understand the true flow of the game from one playthrough alone. The cause of this is actually in the Yin chapter. The first half of the Yin chapter shows the time before Kureha and Utsuki's fusion is complete, while Utsuki is partially self-aware. To put it directly, Utsuki is dreaming. Because of this, phenomena such as suddenly changing areas occur during this time, and Utsuki's mannerisms are like those of Kureha. In contrast, the Yang chapter is made up entirely of things that occur in reality - in other words, it is fact that Sakuya meets up with Utsuki, Douryou and Kureha. Incidentally, the Kuon chapter summarises the events that occur after the end of the Yang chapter.
In the ending, Sakuya leaves the manor with a young girl. This girl can be thought of as Utsuki reborn, but it is not actually Utsuki. Like the way the twins were manifestations of the mulberry trees, she is one also. She is a being that was created when the mulberry fruit born from Utsuki's completion of the ritual sprouted, dwelling within it. In other words, at the same time as the girl being a reincarnation of Utsuki, she could also be said to be a reincarnation of Fujiwara Yorichika and Douryou who were used as materials in the ritual. However, she appears quite different to the twin girls. Perhaps this is because the girl is a special being, or maybe because the mulberry tree in which she dwells is still young - there is no clear answer.