There are multiple alternate dimensions that exist alongside reality, and some are inhabited by sentient and non-sentient entities. Endless multitudes of these alternate planes exist and can be accessed in various ways. There are three main alternate planes that interact with our own, Ilador, Vanaril, and Bagorum.
The Cardinal Beasts
The cardinal beasts are four powerful mythical beast spirits from the plane of Vanaril that crossed over to the mortal realm throuh the goddess root in ancient times. They bestow powerful abilities upon those who find them, and typically are kept with their user at all times.
Possession in the world of Bijutsu no Seken is the process of linking your soul to that of a being in an alternate plane of existence. This link is called a tether and while one can undergo special training to strengthen it, the strength of a tether is largely dependent on the strength of both parties. Through this process, one is able to gain access to the innate abilities of the beings that they're linked to, with more abilities being accessed with stronger tethers.
There are two different types of possessions able to be accessed, physical type possession and domain type possession. Physical type possessions give access to the characteristics and abilities of the being's body, while domain type possessions give access to the characteristics - and even materials - of the environment in which that being rules over. Depending on the strength of one's tether, they may be able to access physical possession abilities, domain possession abilities, or both.
Liminal gates are places where two planes connect and allow crossing between them. These usually allow for just a spiritual crossing, allowing someone to create a tether with a being from the other dimension. However, some liminal gates allow for a physical crossing, such as the Goddess Root opening and letting through the cardinal beasts.
Known Liminal Gates
Below is a list of the known liminal gates and the dimension that they connect to.
Goddess Root - Vanaril
Gate of Alberose - Bagorum
Cidonti - Cidont's personal dimension
Coral Caves - Vanaril, specifically Nydry's domain
The Obelisk - Ilador