Reiryoku - also known as spiritual energy - is an energy source generated and utilized primarily by spiritual or otherworldly entities. Shinigami are the the most famous practitioners of utilizing Reiryoku but all Spirits and Undead generate large quantities of spiritual energy; it is can be said to be superior to Ki which can only exclusively found in physical, biological entities.
Living individuals can generate Reiryoku as well but the amount generated is so insubstantial that for the majority of living creatures it can never be used in any meaningful way. Rare individuals with high concentrations of Reiryoku are exceptional and can become spiritually aware as they grow older; seeing ghosts and other supernatural creatures without assistance. Additionally spirits such as Shinigami can bestow the ability to generate Reiryoku in larger quantities onto living beings. This enhanced spiritual awareness can be passed on to descendents genetically.
Reiryoku is one of the components necessary to create Chakra within the body - but individuals do not necessarily requires large quantities of Reiryoku for this process.
Generated in small quantites inside of living beings (and forming the substance of their souls) and generated in massive quantities in all variations of spiritual beings; conventional Reiryoku' can be found in all living and non-living entities and has various applications. Certain demonic entities known as 'Hollows make use Reiryoku as a basis for their techniques; to increase their speed; and to increase their defensive capabilities.
Shinigami have a range of techniques resembling magic but using Reiryoku as a basis instead of Mana. These are known as Demon Arts or more commonly as Kidō and there are hundreds of named and practiced techniques covered by this. Additionally Shinigami make greater use of Reiryoku in the form of their Zanpakutō - spiritual weapons with unique connections to their own souls.
Beyond both mortals and spirits are Gods who possess a higher form of Reiryoku than all others - except for those who artificially achieve Transcendental Reiryoku on their own though there are few ways to accomplish that. This Reiryoku is very similar to Divine Ki - only occurring when the deity has manifested physically and cannot be sensed by Reiryoku-using entities unless the users deliberately lower their power to be sensed by others.
When individuals - both living and dead - generate massive amounts of Reiryoku they are referred to as having Reiatsu or Spiritual Pressure. This pressure can be exerted on the corporeal world for a multitude of effects, and can be used to suppress other spiritual entities who have a weaker spiritual pressure.
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