The Void

The Void (also known as Ayin, Chaos, Ginnungagap, the Primordial Waters, the Abyss, and Apollyon) is an abyssal void that exists outside of reality. It exists outside of time and space and was the state of all reality before the birth of the Monad (the Big Bang). Even with the onset of creation, the Void still exists in between the various universes, dimensions, and timelines and in a way connects them together. It is certain that at the end of reality and existence dies, all things will return to the Void and nothingness will envelop everything. 
The Void is the very embodiment of nothingness and it is almost impossible to reach. Even if someone were able to enter the void, they would almost assuredly be disintegrated into nothing, resulting not just in death but total and complete cessation of existence with no afterlife or reincarnation to speak of. There are ways through both magic and science to survive in the void without being erased from existence, however, it is extremely easy to be lost in the void which can result in someone never being able to find reality again and being stuck there forever, eventually leading them to insanity and death (making matters worse if their soul still resists being disintegrated, then it is stuck in the void for all eternity and will not reach any afterlife or be reincarnated). For most even spending a few seconds in the void will utterly break their mind and cause them to go completely insane, with only a few possessing the mental fortitude to resist madness and traverse the void. Theoretically, it would be possible for one to traverse the void and enter into different realities and timelines, though it is almost impossible.
There are three known living embodiments of the Void which exist, Khaos (which was enshrined in Greek Mythology), Ayin (enshrined in the Kabbalah), and Abaddon (who was created by and serves Ayin and is enshrined in Christianity). They are embodiments of the void and were both born in it and control it in some way. Thanks to Ayin's meddling, Lucifer can be considered a pseudo-embodiment of the void with the ability to exist in the void and similar powers to the three proper embodiments.