Tartarus

Tartarus is the location where the evil spirits and demons of Equestria are locked away. It is connected to Equestria via a gate that is guarded by the three-headed dog Cerberus. Due to it's similarities to the Christian Hell and large potential for exploration, Tartarus is frequently used to frame plots and create dangerous OC enemies. It is also one of the most discussed aspects of Equestria, as it's role as a hell seems off in the context of a child's cartoon.

Depiction in Fan Labor
In Greek mythology, Tartarus is the dark abyss beneath the realm of Hades where the evil are punsihed for their sins and the dark and dangerous foes of the Earth is kept prisoners, including the Titans. In My Little Pony fan labor, the role of Tartarus is very similar, where it serves as a prison for dark demons and other malicious spirits.

Most of the depictions of Tartarus in fan labor is very similar to the general depiction of the Biblical Hell common in several religions, a burning wasteland filled with magma pools and inhabited by grotesque demons and abominations. However, it tend to differ between wheter or not Tartarus is a place reserved as a prison for evil spirits or a place where evil beings end up after they die, like in the Christian Hell.

The creatures of Tartarus are often described as horrible monsters and mutilateded demons that have little to no remorse or feelings. While some of the demons tend to be cunning tricksters and very manipulative, almost none of the prisoners are shown to have redeeming qualities. The working system of Tartarus varies, but most of the times the demons are prisons who roam the wastelands. Fan works can also include a "Lord of Tartarus" who keeps the dangerous spirits in check. This can either be an OC character, a magical energy field or even Hades himself.

It is also generally accepted that Tartarus is connected to Equestria through a gate guarded by the three-headed dog Cerberus. The exact form of the gate varies greatly, but it is often described as a large structure.

Roles in Fan Labor
Tartarus often serves two main roles in fan labor. It is mainly used as a swearing word, similar to Hell in our world. It is also similarely used in prashes like "I would go through Tartarus for ''you!" and as a metaphor, like "The sauna was hotter than the fires of Tartarus"''.

The other role Tartarus serves in fan labor is closer tied to its role as a prison. Several fics features dangerous demons escaping through the Gates of Tartarus and starting to spread havoc across the land. Sometimes, the main characters even have to venture into Tartarus itself to reach a goal or to defeat a demon.

Cerberus
Similar to Tartarus, Cerberus also hails from Greek mythology, where he guards both the gates to Tartarus and the gates to Hades' realm. It the show, he has the visual appearance of a three-headed black Bulldog.

Cerberus appears in the episode It's About Time, where he leaves the Gates of Tartarus and ends up in Ponyville. While he appears ferocius, Fluttershy quickly calms him down and Twilight Sparkle then tricks him back to Tartarus with a rubber ball, leaving most fan interpretations of him as a rather childish and dim-witted dog, despite his huge responsibility. Also, the show suggest that he is a male and that all of his three heads can think independently.

Most fan fics that directly includes Tartarus often also includes Cerberus. In most of these fics, Cerberus is tricked or defeated by a demon who then escaped Tartarus.

Links and Sources

 * Cerberus on the MLP Wiki.
 * Tartarus on the MLP Wiki.
 * Cerberus on Wikipedia.
 * Tartarus on Wikipedia.