Dungeons & Dragons: Honor Among Thieves is set for release in March 2023, and players are already getting stuck into their campaigns in anticipation of the cinematic launch. Within the context of Dungeons & Dragons lore, there are multiple Planes of Existence that contribute to the makeup of the multiverse.
How that cosmos of possibilities will play into the movie adaptation remains to be seen, but Dungeon Masters will be very familiar with the most dangerous Planes of Existence within Dungeons & Dragons, that can completely throw a campaign off the rails. The Planes of Existence are so important but these specific D&D inclusions are more chaotic than most.
The Material Plane
The Material Plane is where all Dungeons & Dragons campaigns normally begin. It's the basic reality, where quests are conducted, and dungeons are adventured through. There are an infinite array of worlds within the Material Plane, but it's a neutral space, with wondrous and dangerous areas.
The worlds vary and what could happen to a party depends on the Dungeon Master's use of the Material Plane. Some campaigns never even move out of this realm, but the influence of other Planes of Existence can often be felt in Dungeons & Dragons' Material Realm, especially if beings from these alternative areas cross over.
Ethereal Plane
The Ethereal Plane is a dangerous location for those who don't know how to navigate it. It feels almost endless, with the cloudy, foggy landscape mirroring other Planes of Existence, with the Ethereal physically tying itself to these areas. Thus, there is a layer of unpredictability linked to its geography.
The Ethereal Plane will pop up within a campaign as a way of traveling between other Planes of Existence. Only the most experienced will be able to use this without any issues. There may be fewer monsters and threats than the Material Plane, but poor planning could mean that the player ends up in a reality they don't wish to be on, opening up other dangers.
Astral Plane
The Astral Plane is very similar to the Ethereal Plane in its usage, acting as another way to travel between other Planes of Existence. While the formerly mentioned Plane is complex to navigate, the Astral Plane is on another level and could be corrupted.
It's a land of thought and dreams, with players traveling through in their Dungeons & Dragons adventures not as physical entities but simply through their minds. That can have terrible effects on those who are unable to control this method of movement, and the endless nature of the domain means that someone could be forever lost in its vastness.
Elemental Planes
The Elemental Planes have been grouped together here because they are all equally dangerous in one way or another. The Elemental Planes each represent something from the Material Plane: Earth, Air, Fire, and Water. Thus, each of these landscapes is an extreme variation of those elements.
A realm purely of air might be impossible to survive for those who can't fly, while it's obvious that a player could drown on the Water Plane or burn on the Fire Plane. The threats of the elements are heightened on these planes, making them difficult lands to explore. The Elemental Chaos that takes place constantly is non-negotiable and there are vast areas that players will have no knowledge of.
Demiplanes
Demiplanes are much smaller in scale than a traditional Plane of Existence. They are not built into the cosmos in the same way, arriving erratically and accessible through small tears in the fabric of reality. Each Demiplane is totally different and obeys its own laws of nature.
Whether they are naturally forming thanks to a mutation in the dimensions, or could even be created by a player in certain circumstances, the Dungeon Master often has massive levels of control over how unpredictable a Demiplane is, making this potentially one of the most dangerous Planes of Existence in Dungeons & Dragons.
Gehenna
The rest of the Planes of Existence discussed are all a part of the Outer Planes, specifically the more nefarious Lower Planes. Gehenna is volcanic in its geography and one of the bleakest locations that a party could have the misfortune of questing in.
This Dungeons & Dragons Plane of Existence is neutral in its alignment but absolutely deadly to try and explore solo. It's a combination of 4 layers, each of which is just as difficult to climb through thanks to its rocky terrain and fiery blasts that could cause a volcanic explosion on a massive scale at any moment. This is not a good Plane for first-time player characters.
Carceri
Carceri has also been titled Tarterus, which would be a name familiar to fans of Greek mythology. A hellish realm that has links to the underworld itself, it's built of 6 layers and boasts realms that trace other mythologies associated with the underworld and afterlife.
Carceri houses the worst of the worst, acting as a kind of prison that sinners are sent to, in order to atone for their crimes. No party would wish to willingly cross into this realm, that's home to murderous terrors, cultists, and those who committed some inhumane acts. No one can be trusted.
Gray Waste
This Outer Plane is home to a faction war between the deadly demons and devils that want to claim the territory for their own and is the Plane of Existence that Hades watches over. It's another area associated with the mythology of Hell itself, which slowly drains any visitor of their most redeeming qualities.
Surviving this area as a non-monstrous creature would be incredibly difficult. Not only is the barren landscape hard to inhabit, but the consistent conflict among some of the evilest of beings is a hard narrative beat to defend against and can throw any campaign into further chaos.
Nine Hells Of Baator
Races that can't even be identified in the rule book can be found in the Dungeons & Dragons Plane of Existence known as Baator or the Nine Hells. This sinister area is the embodiment of Hell itself, hence the name, and is home to the torturous devils that will never allow their victims to escape.
Every single layer of this Plane is deadly to visitors, but their rules operate completely independently of the others. Dungeon Masters might find inventive ways to kill off players in these lands, considering the Nine Hells are designed to dispatch of the pure of heart. From a geographic perspective alone the terrain is uninhabitable.
Abyss
While the Nine Hells might house the devils, the Abyss is where players might find the demons. It's just as inhospitable and full of deadly creatures that any party would find difficult to combat. It's also where a campaign could be based forever, with only a doomed ending to cap off the experience.
The Abyss is an infinite area. While there are claims that there are 666 layers, in lore, no one has returned after the first couple of hundred. If a party wishes to dig deeper into the Abyss they probably will never surface again, making it the most dangerous realm full of horrific secrets.