The Multiverse
Disclaimer: Due to the player party's sever lack of knowledge and understanding of the topic at this time, information here is severely redacted.
Multiversal travel, although forbidden and sometimes even prosecuted by the caretakers, has always been common and beings who managed to travel through the Multiverse are commonly referred to as Spirits.
Multiversal travel in general requires an immense amount of Mana-Control to even attempt and is believed to fail in the vast majority of cases, with a fatal result.
First Gate - Deconstruction
The first stage of multiversal travel, the deconstruction of the physical body in subatomic fashion. This is an extremely high-risk procedure for anyone attempting it for the first time and believed to be the stage the vast majority of beings attempting multiversal travel fail to survive. The traveler is aiming to become one with the cosmic Mana of their own universe and later push through a rift in space. Most are believed to fail this stage not necessarily due to lack of Mana-control, but due to being unable to retain their personality and individuality after their deconstruction - essentially failing to keep their Mana form from dispersing - after which even any kind of rescue from third parties becomes impossible.
Second Gate - Travel
The second stage of multiversal travel is the actual travel through subspace itself. Navigating subspace, however, is a complete black box. No one who successfully did so has any memories of it. As a result, the second gate is more of a theoretical construct to explain the occasions of beings who succeeded in the deconstruction but never arrived in spiritual form.
Third Gate - Reconstruction
The third stage is the reconstruction of a physical body after successfully arriving in a different universe. However, since the Mana of every universe is entirely unique, the differences to their native Mana cause most beings to not only fail to keep their individuality, but from taking physical form altogether. These "Gate2-Spirits" are also known as Lesser Spirits. The incompatibility that makes the Mana hard to control in the first place also keeps the being's Mana from dispersing, blobbing it so to speak. In contrast, those passing the third gate are considered Greater Spirits by academics - a term tied to the fourth gate for about anyone else.
Fourth Gate - Mind
The fourth and final stage of travel is the manifestation of the traveler's original mind. This is by far the hardest gate to pass, proven by the sheer amount of magical beasts who passed the third, but failed at this final stage. Greater Spirits commonly refers to those travelers who managed to keep their mind. There are only a handful of known Greater Spirits, all immensely powerful and feared.