Spirits

This has gotta be the most frustrating thing anyone's ever tried to research. Seriously, how do you research something you can barely even interact with?!

— Unknown Inquisitor

Spirits are one of the most enigmatic things in the world.
The existence of a spirit appears to be a corollary of the existence of something alive, or perhaps it's the other way around. Whatever the case, it is known that for everything alive, there exists a corresponding spirit. All these spirits are located in the Spirit Realm¹.
The purpose of spirits is largely unknown. However, they are critical to the being they correspond to; destroying a creature's spirit² will invariably result in the death of that creature³. This does not work in reverse; killing a creature will not result in the destruction of its spirit.
Spirits have a fairly simple anatomy. They consist mainly of a pale bluish substance that appears similar to a gas, but in truth seems to be completely immaterial, easily passing through all materials. This material is surrounded by an irregularly-shaped shell consisting of a fragile crystalline material that does not seem to exist outside of the Spirit Realm, and appears to be the only thing the "gas" inside spirits cannot pass through. The spirits of more advanced beings also contain a soul shard, and these beings always have a matching one inside their heart. These soul shards allow those beings to utilize magic, and are the only known thing to be able to create a connection between the Spirit Realm and Verta or Aead⁴.
Ideas on the purpose of spirits are numerous. A common belief is that, when a being dies, that being's consciousness is transferred into its spirit. This belief is not absolutely unfounded; careful analysis has detected certain differences between the spirit of a dead being and that of a living one.
Spirits are also known to be able to control magic, and do so far better than their corresponding beings' minds can. This is evident in people with the ability to use shadow magic, as such magic is always shaped by the spirit before being released from the Spirit Realm. However, this is the only case in which spirits are known to shape magic directly.
Many things are still unknown about spirits. The Inquisition and other organizations of mages have been making efforts to learn more about them. However, owing to the difficulty of interacting with or even observing the Spirit Realm, progress has been slow. It is uncertain if the whole truth behind spirits will ever be known.

¹Except for the secondary "pseudo-spirits" of Great Powers, although whether or not they can truly be classified as spirits is debatable.
²This is possible using incredibly powerful black magic. However, even then, this has only been successfully performed on simple creatures.
³This occurs even in creatures without soul shards, despite the fact that no connection between the Spirit Realm and that creature's world should be possible without one. Of course, the mere fact that spirits appear as life does also seems to contradict that notion. Further research on the capabilities of spirits to connect the Spirit Realm and the physical worlds is ongoing.
⁴It should be noted that it IS possible to observe the Spirit Realm using magic. However, all currently known ways to do this require the usage of a soul shard in the ritual.