Phase (月相 Tsuki-shō) are a species of immortal human beings. They were created by the Moon God Tsukuyomi upon the appearance of the first Oni. They possess great strength and magical powers, which they use to combat Oni. Phases are capable of using magic which they can develop into a Lunar Art. Which is crucial when fighting Oni.
Phases were born during the Heian Period, back then, the first Oni started appearing in Tsukihara and started to torment and kill innocent humans. Humans lived in fear, as Oni were most active during the night, they hid in their homes. However, they would barge in people's homes and kill them. People pleaded and prayed to the Gods, wishing for protection. Hearing their pleas the God Tsukuyomi gifted certain people with powers. These people were close to the moon, due to the fact that they were born on the night of a full moon. Similarly to the Oni, these people would get stronger during the night, powered by the Moon.
These people would train their powers and learn how to fight, eventually becoming strong enough to fight against the Oni. Over time, they had slain enough of them that peace returned to Tsukihara. Oni became afraid of the ones who would later be known as the Phases. Now afraid to come out at night, fearing that they might kill them. For months, Tsukihara knew peace. The Oni went to other places in Japan in order to feed on humans. Unfortunately for them, Phases were everywhere. As Tsukuyomi gifted his power to several people across Japan. Oni started to dwindle, and humans were finally protected.
However, hope came for the Oni in the form of Katsurou Chisō. A young man who had become a powerful demon and came to Tsukihara to give his powers to the Oni. Thanks to him, they became more powerful. Their blood became a weapon, now capable of poisoning humans and Phases. The Phases then started looking for ways to become more powerful. They prayed to Tsukuyomi, who gave them the ability to use Lunar Arts. Phases were now capable of using magic and created an art that combined magic and combat. Katsurou created more and more Oni. The Phases were outnumbered as more Oni were created.
One of the Phases, Mangetsu, went to find other Phases and suggested that they should team up. Together they could take down a lot of Oni and prevent more death and destruction. That was when groups of Phases were made. They gathered together in what would become the Lunar Mansions. Where Phases would train and devise plans against the Oni. Together, they could take down more of them and counter them, despite them getting stronger thanks to Katsurou.
Mangetsu became the leader of the Phases of Tsukihara. Under his leadership, they became one of the most powerful group of Phases of Japan. He remained the leader of the Phases for the next periods to come. Eventually, him and the other Phases faced Katsurou in a bloody battle. Mangetsu managed to hurt him and attempted to end his life. But other Oni came to his defense and protected him. He had no choice but to retreat. The Phases then came with a way to differentiate each other, by naming themselves after the Moon phases. Due to that, Phases came and went, taking the role of the previous Phase before them. Although it would often take a few centuries before happening due to Phases being immortal and becoming more powerful as they age.
In current times (Heisei Period), the Lunar Mansion of Tsukihara is composed of Mangetsu, Shingetsu, Mikazuki, Misokazuki, Komochizuki, Imachizuki and Yuta Tsukikawa. Together they form the main fighters and are the most powerful. Although Yuta is still in training. Mangetsu met Shingetsu during the Meiji Period, and the rest during the Taisho and Showa periods. Each of the Phases have pupils who are trained into becoming Phases. Some of them are even trained into becoming the ones to take the current Phase's place. These pupils will then become the Phases who will complete the Lunar Mansions which can be composed of eight members. Corresponding to the number of Phases the Moon has.
Phases are still in an ongoing war with the Oni, whose number grows each night. Mikazuki for the first time in ever since he became a Phase took a pupil under his wing. Yuta took residency inside of the Lunar Mansion and is in training. He aims to aid Mikazuki in his mission to create a cure to the poisonous blood of the Oni. Phases have grown much stronger since then, but unfortunately the same happened to the Oni. The Oni plan to rid Japan of Phases forever and feed on humans to get stronger. The war gets worse as a new, more powerful Oni joins forces with Katsurou to help in his plan to kill all the Phases. Mangetsu decides that the war has to end and speeds up the training process of the pupils in order to have a complete Mansion. He urges the other Mansions of Japan to do the same, in order to prevent the extinction of the Phases.
Phases are normal for the beginning of their life, their powers start developing early in life. Phases age normally up until their late 20s, when the human brain finishes development. That is when they reach immortality. The senses are the things that develop first, they slowly start to gain more in terms of strength and develop their powers. With training, they can then learn how to summon the elements and use magic, which will later contribute to the creation of their Lunar Art. Phases reach peak human condition after puberty, becoming faster, stronger and also healing faster. They look completely human and don't look any different from them.
After reaching immortality, phases get stronger, each full moon, their powers get strengthened. It has become a habit for them to train their powers and skills during the night. Phases don't possess visible weaknesses, they can't be killed in a normal way. They are capable of healing from blade wounds and even bullet wounds. Wounds caused by demons that are not deep can heal quickly. However, demon blood is harmful to phases, as it poisons them. Phases can purge demon blood from their body quickly contrary to humans. But a large amount of blood will eventually poison them and kill them.
Phases look normal and thus blend well with humans well contrary to demons (mostly demons that are of a higher rank). However, phases stop aging. Certain humans notice how the years don't affect them at all, even if everyone around them starts aging (although these humans are made to promise to not reveal the truth of their nature). Everything about the phases look normal on the outside.
As they age, phases start to develop certain abilities such as extrasensory perception. Being able to detect the presence of others, and with time, being capable of detecting the presence of demons. They have a heightened awareness of danger, a Phase as old and powerful as Mangetsu can make the difference between the presence of a phase, a human and a demon. He can also predict his enemy's moves and counter surprise attacks. Young phases can develop these powers if trained enough. Elder phases are also capable of fully developing psychic abilities such as telepathy and telekinesis.
- Superhuman Condition - Phases are at peak human condition. Their senses are heightened, as they are capable of hearing and seeing things from miles away and have a heightened sense of smell. Which is useful in tracking Oni. Some phases have a heightened sense of touch, which is quite rare. It allows them to hear other people's thoughts upon touching them or feel others' pulse/heartbeat or blood circulation. Phases are at peak physical condition. Being able to run at lightning speed, have enough strength to fight with Oni in a hand to hand combat and having tough skin. Phases heal at a much faster rate than normal humans and are capable of fighting off demon blood which is poisonous. They can't be hurt easily by weapons and humans, and can only be hurt by Oni and other phases.
- Magic - Phases are capable of using magic, mostly using it to summon elements. They are capable of using spells and incantations. Spells such as Bind to chain demons, or Sleep to put people to sleep or Banish which exorcises demons. Phase magic can be used in a lot of ways, Komochizuki often uses it to levitate. Mangetsu uses magic to scry for Oni's presence, or create a barrier to fight Oni without causing collateral damage. Phases can also write spells on pieces of paper that they put in certain places (such as the entrance of the Lunar Mansion to conceal it from the outside world and Oni).
- ESP - Phases can develop psychic abilities, using these abilities to detect demonic presence or the presence of people in general. They can detect imminent danger or predict it. Mangetsu has trained himself into detecting and predicting Oni. Phases can use telepathy to communicate, or use it to hear people's thoughts in order to know if they are telling the truth. ESP can also be used to determine the power level of an Oni. It also gives the user hyper awareness to certain things. Stronger users can even see through things, including human beings and Oni and use divination.
- Semi-Immortality - Phases are mortal for most of their lives until they reach their 20s where they become immortal. Before that, it is possible for a phase to be killed. Once they become immortal, they develop a resistance to any illness. They cannot be affected by things such as temperature changes (they can't get hypothermia or die from heat). Their skin gets tougher which makes it unable for them to be injured by blades or bullets. They also develop the ability to heal faster.
- Oni Blood - Blood from Oni is the only known thing capable of hurting Phases. If a phase gets injected with Oni blood, they will get poisoned. They will become tired, sick and their veins will turn black. Most phases are capable of fighting off the infection but a large dose of Oni blood might be enough to kill them. If not treated immediately, the blood cannot be extracted from the phase and will spread to the rest of their body.
- The creator of the manga was inspired by Kimetsu no Yaiba's breathing styles when creating the phases. They are quite different from the demon slayers as they use actual magic and spells against Oni.
- During the Kamakura period, all phases were required to take on the name of the phase they represent in order to conceal their true identity. Phases will sometimes be names after the phase themselves (exemple: Mikazuki).
- Phases were made immortal, but their immortality only activates once they reach adulthood. It was made this way so that their childhood could be normal. However, phases from powerful families could develop powers and regenerative abilities at an early age.
- Being a phase can be thought, but most phases are born and inherit the name from the previous family member. As they are immortal, they tend to be phases for at least a few centuries before passing down the title.
- Phases train together under the tutelage of older phases because it is easier for the older phases to protect the ones who haven't discovered their powers yet. In the ancient eras, Oni would attack children they believed to be potential phases. Lunar Mansions were created to protect phases and act as training grounds for potential phases.