The first question to ask about any plane is not what it looks like. It's how it works. What are the rules of existence in this place?

Does time move differently? Does magic behave differently? Is gravity reliable? Does emotion function normally, or does the plane alter how creatures feel or think?

These mechanical questions are more important than the visuals because they determine what it actually means to be there. A plane where emotions are dampened changes every conversation your players have. A plane where magic is unreliable changes every encounter. A plane where time moves at a different rate changes the stakes of every side quest that takes you there.

Equarium has four planes, each with its own set of rules. The material world is Equarium itself. Eterisk is the Other Side, a parallel plane where colors are muted, gravity is lighter, sounds are muffled, and emotions are dulled. Souls that have escaped or avoided their destined afterlife drift there. Magic is weaker in Eterisk. Spells cast there have reduced effects. The whole plane feels like a world seen through frosted glass, because that's essentially what it is. Then there's Perditious, the Five Chambers of Hell, which exists because a god literally sacrificed his heart to imprison a rebel deity, and that heart became the hells themselves. Every detail of Perditious follows from that origin. Then there's Magnoris, the floating island realm built from the wreckage of a continent shattered by war, where one day equals four days in Equarium and your jumping distance doubles.

Each plane works differently because each plane exists for a different reason. The rules come from the lore, not the other way around.

The best planes in any multiverse serve as thematic mirrors or contrasts to the main world. They amplify something the setting is exploring.

If your main world is about the cost of war, maybe one plane is populated entirely by the echoes of battles that happened there, replaying endlessly. If your main world is about the abuse of power, maybe one plane is perfectly ordered and completely empty because its rulers' logic drove out every living thing.

Eterisk reflects Equarium's preoccupation with death and the unresolved. Souls that didn't make it where they were supposed to go. Events that left echoes. The plane is a record of what Equarium couldn't process. Bodies of water in Eterisk show distorted reflections of Equarium. Major historical events replay as ghostly loops. The plane is literally the world's unfinished business. That thematic coherence makes Eterisk feel like it belongs to the same setting rather than being a random addition.

If your players can jaunt between planes without consequences, the planes stop feeling distinct. The cost of travel is what makes planes feel like genuinely different places rather than fast travel points on a map.

Cost can take several forms. It can be mechanical: getting to Magnoris means arriving at a random location and possibly losing four days of real-world time for every day you spend there. It can be narrative: entering Eterisk means your emotions are suppressed, which affects how your character processes everything that happens to them while they're there. It can be irreversible: some things you bring back from Perditious don't leave clean.

The cost is also what creates decisions. Players who can teleport to any plane at will have no reason to think carefully about when and why they travel. Players who know that travel has consequences start thinking strategically about the multiverse, which means the multiverse is doing its job.

One of the most interesting things you can do with a planar system is deliberately destabilize it. Instead of planes being stable, accessible places with known rules, what if the connections between planes were unpredictable?

That's what the rifts do in Equarium. They're not portals you plan a trip through. They're tears in reality that appear randomly, can connect to any plane or any alternate timeline, warp the physical environment around them, mutate creatures, destabilize magic, and can vanish without warning while you're still on the other side.

The rift system means that the multiverse comes to Equarium whether the characters want it to or not. That completely inverts the usual planar travel dynamic. Instead of your players choosing when to engage with other planes, the other planes intrude. That keeps the threat active and the world unstable in a way that feels true to the setting's grimdark tone.

You can apply this principle to any campaign. What if the border between your main world and its mirror isn't a door you open, but a wound that keeps bleeding?