My long running 5e campaign is set in a multiplanar setting. Lots of small, pocket worlds, insulated from each other by the astral plane. There is both a plot and a playstyle justification for this, but that is not important right now. What is important is the hook:

My players were the crew of a first voyage of an experimental vessel. A ship, but the bow is an enormous conical point, with a silver plating. And it’s sitting in a dry dock 50 miles from the ocean. Inside that cone is the engine room, containing a massive crystal. 5 feet tall, like a glass prism filled with fog.

This ship can travel between the planes. It first slips into the border ethereal, and the crystal inverts, now showing a distorted image of the plane you left. It can travel a while here, and while it can be used for interplanar travel, the purpose is to sail to the edge of this world’s ethereal plane out into the astral plane beyond. This is how that works, mechanically:

Doing it Right

The engineer primes the crystal. It’s not difficult, just a few switches. It begins to charge. After one minute, or 10 rounds, the engineer throws the switch that activates the crystal.

The ship will shift into the border ethereal of the current plane. This is a 1 for 1 with the plane itself, but everything is made of fog. The engine can move the ship here. You can travel to other places on the plane this way, but the potential is much greater.

If you sail far enough in the ethereal, you get to the deep ethereal. This place no longer maps to the plane you left, you can no longer shift back to the material. Eventually you’ll reach the border of this place, where the fog drops off a cliff waterfall, and the droplets make an infinite vista of rainbow.

The mortal mind can’t comprehend the transition. Rainbow light will fill your eyes, and you’ll come to floating in the astral plane. You’ll be able to see a representation of the plane, like a planet from outer space. Now you can sail out and find another plane. Sailing into it brings you to that plane’s deep ethereal. From there you can venture into the plane itself.

Art by Noah Bradley

Doing it Wrong

The ship takes one minute, or 10 rounds, to prime before it can shift between the material and ethereal planes. If the switch is thrown early, there is a chance of mishap. Obviously, when travelling under no pressure, there is no chance of mishap. Only when circumstances conspire against the crew must they take risks.

But why not simply leave the engine primed, so that you can travel on a whim? That is because of Instabilities. The crystal is eminently magical and indestructible to normal means, but its internal magic can be thrown askew. For each round you hold the charge, it gains 1 Instability. Instability is hard to remove, taking a day of downtime and 50 gp to remove a single count. In addition, if the crystal is ever damaged, it gains 1 Instability for each 5 damage it took, rounding down.

If Instability ever reaches 10, the ship begins to prime itself. You cannot stop it, and if you knew what would happen if you did, you would have designed it the same way. The ship will automatically shift after the requisite 10 rounds. There is a near guarantee of a mishap, so prepare yourselves. Luckily, this event clears up all Instability.

When the ship shifts, roll 1d10 – (The number of rounds you’ve spent priming) + (The current Instability). If there is any Instability there will be a chance of a mishap, even if you wait the full 10 rounds. If the roll is 0 or lower, everything went well. The higher you rolled, the worse it was. If you roll a 1 or greater, roll a d6 to see what type of mishap occurred. Then go check the appropriate table.

1d6 Type 1 Misplaced 2 Disoriented 3 Overclocked 4 Stalled 5 Damaged 6 Strange

Misplaced. You appear on the material plane a certain distance away from where you were, or where you intended to be.

Roll Misplaced 1 100 feet in a random direction 2 1000 feet in a random direction 3 1 mile in a random direction 4 1 days travel in a random direction 5 2 days travel in a random direction 6 3 days travel in a random direction 7 1 weeks travel in a random direction 8 2 weeks travel in a random direction 9 Any random location on the same plane 10 On a random, different plane

Disoriented. The ship is oriented incorrectly, or objects and people are misplaced.

Roll Disoriented 1 Turned 180 degrees 2 Rolled 90 degrees (Sides down) 3 Pitched 90 degrees (Bow straight down) 4 Rolled 180 degrees (upside down) 5 Everyone onboard is 100 feet away, together 6 Everyone onboard is 1 mile away, together 7 Everyone onboard is 10 miles away, together 8 Everyone onboard is 100 miles away, together 9 Everyone onboard is in a separate, random location on the same plane 10 Everyone onboard is in a random plane, in a random location, separately.

Image by AsherGeist

This is an example of Overclocking gone very, very wrong

Overclocked. The ships engine is carefully calibrated to teleport only the ship. With this mishap, extra area around the ship is carried with. Everything that is that close to the ship is carried with. Large structures can be cut apart by this

Roll Overclocked 1 5 extra feet 2 50 extra feet 3 50 extra feet, plus everything on the other side is swapped as well. 4 250 extra feet 5 500 extra feet 6 500 extra feet, plus everything on the other side is swapped as well. 7 2500 extra feet 8 1 extra mile 9 1 extra mile, plus everything on the other side is swapped as well. 10 10 extra mile, plus everything on the other side is swapped as well.

Stalled. You don’t teleport at all. The engine stalls for the listed amount of time. Then, it fires. You don’t know how long it will be, and you can’t use the plane shift until after it happens.

Roll Stalled 1 1 round 2 2 rounds 3 3 rounds 4 4 rounds 5 5 rounds 6 1d6 minutes 7 1d6 x10 minutes 8 1d6 hours 9 1d6 x10 hours 10 1d6 x10 days

Damaged. The crystal is damaged. It will take time and money to fix it. This will not fix any latent Instability.

Roll Damaged 1 1 minute and 10 gp 2 1 hour and 10 gp 3 1 day and 10 gp 4 2 days and 20 gp 5 4 days and 40 gp 6 6 days and 60 gp 7 10 days and 100 gp 8 20 days and 200 gp 9 50 days and 500 gp 10 100 days and 1000 gp

The ethereal is rife with spirits

Strange. Some things cannot be predicted. Many of the mishaps that occur during planar travel do not fit a clean category.

Roll Strange 1 Everything that plane shifted is unaffected by gravity for 1 minute 2 All food that plane shifted becomes ethereal, and inedible. 3 Shifting seems normal, but you soon discover that a week passed in the blink of an eye. 4 Everyone on board gets possessed by good natured spirits, who take the bodies for a spin for 1d4 days. You have no idea what they do. 5 One person (prefer NPCs) that shifted becomes phased out. They didn’t plane shift with you, and every time the ship shifts they shift. This ends when they are brought to the same plane as the ship. 6 One person (prefer NPCs) is tossed out into a random part of the astral plane. They’re okay, but will be very very hard to find. 7 A permanent portal between the ethereal and the material is ripped open. 8 Everyone on board gets possessed by hostile spirits, who take the bodies for a spin for 1d4 weeks. You have no idea what they do. 9 Shift to a random plane. A permanent portal between that plane and the material plane you were just on is ripped open. Strife is likely 10 The person closest to the crystal gets absorbed into it. They now control it, but cannot ever regain their body.

Mishaps in the Wrong Places

If you try to prime or activate the crystal in the Deep Ethereal or the Astral Plane, the switch will not move. So we usually won’t need to worry about mishaps out there. But, due to instability sometimes firing the ship without input, it can happen. If it does, this is the table to check:

Roll In the Wrong Places 1 Shift between the Border Ethereal and the Astral Plane. If you’re in the Astral, choose the nearest Border Ethereal. Everyone make a save versus crushing headaches and vomiting. 2 Shift to a random location in the nearest Border Ethereal. Everyone save versus unconsciousness. 3 Shift to a random location on the nearest material plane. Everyone save versus unconsciousness. 4 Shift to a random location on a random material plane. Everyone save versus unconsciousness. 5 Shift to a random location on the nearest material plane. The ship is splintered to bits. Everyone save versus unconsciousness. The crystal and it’s mechanisms can be salvaged 6 Shift to a random location on a random material plane. The ship is splintered to bits. Everyone save versus unconsciousness. The crystal and it’s mechanisms can be salvaged 7 Every person on board the ship ends up separately on a random material plane. They save vs unconsciousness. The ship itself also lands on a random material plane 8 The ship shifts back to the nearest material plane, but the people on board are thrown to random locations about the Astral Plane. 9 Time travel 100 years into the future, but don’t move in space 10 The crystal writes itself out of existence. Everyone on board saves versus death, as their bodies visit every plane, some simultaneously. Those that survive are left floating in the astral plane. The dead are scattered, torn asunder across dozens of planes. The survivors can now travel between planes at will, as an action.

Sabotage and Intention

Is there any way to exert control over these mishaps? The severity, no. But there is a way to control the type. If you’re in the room with the crystal, you can try to choose the type of mishap. Add 1 Instability, choose the type of mishap you want, and then roll a DC 20 Arcana check. If you succeed, that is the mishap table you use.

If you fail, that type and two random other types become unavailable. Make a DC 15 Arcana check to choose between the remaining three. You’re scrambling now, trying to make the best of it.

But what if you want to cause a mishap? We’ll assume you are either an enemy of the ship’s owners or extraordinarily desperate. But still, shunting 10 miles of an enemy kingdom into the ethereal plane could very easily be desirable. To intentionally trigger a mishap, you simply need to get instability to 10.

Overpriming it is a simple solution. Simply leave it running for an extra minute, and then the ship will fire off, causing a mishap. In that case though, you’re simply rolling 1d10 on a mishap table, there’s a good chance the result isn’t as catastrophic as you desire.

Damage can get you above 10 Instability, almost guaranteeing a horrible mishap. Even after it begins auto-priming at 10 Instability, additional damage will continue to raise the number.