Adumbra is the name for both the extra-sensory realm and the ability to sense and experience it. Everything we know of it comes from old Windas legends who once knew of the Truebloods. Some animate or inanimate things can exist in Adumbra but not in our realm, and intentionally or accidentally interacting with them can result in sensations such as inexplicable chills, hair standing on end, feeling watched, or sudden sensations of nausea or dizziness.
Adumbra is allegedly described as being akin to opening a "third eye" which allows for a different type of sight. Adumbral sensing can be turned on and off at will, but cannot be accessed unless one is a Trueblood themselves. Some scholars speculate that some people do actually have an extremely small amount of adumbral sensation, which would explain humans sometimes feeling the aforementioned symptoms without any discernible explanation.
The Dark Realm
According to these people, while in Adumbra the world looks physically much the same, but celestial bodies such as the sun, moon, and stars shed no light; rather, light comes from living things such as humans, animals, and plants, and glows a deep-blue purple in the otherwise dark realm. This light also is said to emit a deep hum. As stated, much of the world is physically identical, but extra objects and beings may be found in Adumbra, completely undetectable without adumbral sensing. It is also said that other senses are muted while in Adumbra as well: olfactory senses like smell and taste are very muted, and sounds are muffled and sound shaky or unstable. Additionally, certain materials like water and metal appear very different in Adumbra, though it isn't understood at all, as the old tales make little sense in the context of modern science.
Realm of the Dead
Some scholars speculate that Adumbra is a realm where dead souls rest, as Adumbra itself is apparently much more populated than our realm. This implies that either Adumbra has a large number of native inhabitants, or that some people -- the dead, for example -- can pass into Adumbra, but cannot return to our realm again. Many agree that both possibilities are probably true, and some speculate further that going to Adumbra, or spending too long in Adumbra, will kill the person. Legend has it that the Truebloods vanished in this manner, either by dying and thus can only be found in Adumbra, or by going to Adumbra and then refusing, for whatever reason, to return to our realm.