There are many different areas, supernatural and not within Greendale. One, though, is filled with human souls. This is what we all know as Mortal Limbo. There are so many different aspects, and this is also one of the important ones. After all, this is where Sabrina learns that her mother's soul is, and that she had not found peace. Mortal Limbo is loosely based of the Limbo that many of us are very familiar with. It is considered an edge or a boundary. Just on the edge of hell, in some beliefs. And those who are there, have died in original sin and do not deserve the damnation of hell. It is believed that hell is split into three different sections, Hell of the Damned, Limbo of the Fathers or even known as Patriarchs, and Limbo of the Infants. Limbo of the Fathers (Patriarchs) is where those who despite their sins that they have committed died in the friendship of God, They will remain within the realm until Jesus frees them. Limbo of the infants is the one where the babies that are born who are not baptized go.
This is the place where, unbaptized babies go, or those who die before their time. This realm is at the edge of death. Meaning, the souls there, are stuck. Lost in ways, that they do not understand or even realize. This place is impossible for witches to cross over into, due to the fact witches have their own. But Sabrina, was able to cross into it, due to the fact that she is part human. Souls that are stuck here, are hunted by a monstrosity known as the soul eater. And, well, you can pretty much figure out what happens to those it catches.
There is a threshold within the Greendale Woods that allows those who are living to cross over into Mortal Limbo. When Mortal Limbo becomes a thing we see in the show. It is covered in a dense fog. Souls wander lost and confused. Sabrina is seen to cross her mother's soul while she is there trying to get Thomas Kinkle’s soul. But, as we all well know… she did not succeed in bringing Thomas’ soul back, the soul eater consumed his soul just as the two of them got to the exit of mortal limbo.
A soul eater is what lives within the realm. They are said to eat the souls of the people who reside within the realm. It is believed by some, at least, that soul eaters are men, who were cursed by witches and due to this curse they have to eat the souls of humans to survive. But in Greek Mythology, these beings are spiritual and not humans. There is not much more to say or know about mortal limbo. But, despite all of that, it was a fascinating thing to watch and learn about. And sad at the same time, to know those souls that are there don't even know they are there, that they don't or won't ever get the chance to find peace.