Are there published encounters that send you into an illusion or dream state?

Question

Our DM in a recent adventure had us transported into a matrix like illusion when we used a portal. We thought we were traveling back to Undermountain from an orbiting satellite that was occupied by Githyanki, but instead we ended up in Waterdeep. Only, to find out after some time, that something was not right about Waterdeep, and in the end we found out that our bodies were actually held in some Matrix-like (as in the movie) pods operated by Mind Flayers, and what we experienced was just a very detailed illusion or hallucination. (And of course, the question then became how to escape).

I found this was a cool twist, but I also suspect that this effect is not part of the normal adventure, because for one, the DM has heavily modded the published adventure to make it more exciting, and secondly, in all my years of DMing, I have never run into a portal that did not just transport the body but also sent you to an illusion or dream state. But who knows -- I certainly have not read all the official modules that have been published over the years. This made me wonder - was anything like that published? I'm mostly interested in unusual portals, but other trigger effects that let you end up in a transition to another environment that is an illusion without you realizing it are good too.

Anything from TSR, WotC, including magazines like Dungeon or White Dwarf or novels from authors like Greenwood, Gygax or Salvatore would be a viable source.

Answer 1

SPOILERS AHEAD

X2 Castle Amber (BECMI, Expert Level Adventure - 1981)

In room 50, the alchemical laboratory,

...concealed vents will open and begin to fill the room with a black powdery cloud which moves as though it is alive...Every round a character breathes in the fumes or comes into direct contact with the powdery cloud, the character must save vs. Spells or fall asleep...This particular alchemical preparation can cause exceptionally vivid dreams (or nightmares) and has the magical property of possibly making those dreams (or nightmares) come true.

The module asks the DM to make up the dreams caused by the trap, but gives a number of suggestions, many of which involve the players believing that their PC has been transported somewhere else and that what is happening to them is real.

As an example:

A dream that all the plant life in the world suddenly gains demonic intelligence and plans to take over the world. The character is the only person who knows the truth. Fortunately he or she discovers an ancient artifact which, when destroyed, returns the plants to normal. Unfortunately no one will believe the PC’s story.

One of my players' PC's in the 80's had this dream, and to this day he is still bitter that no one believed her.

One of the dreams actually physically transports the PC to another location, where they remain out of play and dreaming unless and until the rest of the party can find them. This most closely matches the original request for a portal trap that simultaneously affects PC's bodies and minds.

A dream of a strange land called Averoigne...The character will disappear from the room and will be out of play, but will awake in the clearing on the far side of the Gate of the Silver Keys at just about the same time that the party actually arrives and opens the Gate.

The module does not say how long the characters sleep in the laboratory, only that regardless of the length or complexity of the dream, they all awake at the same time. Presumably this would give the Amber family, were they still present, the chance to collect and imprison anyone who had intruded on their laboratory.

The Illithiad (Second Edition, 1998)

According to TV Tropes,

The Illithiad has the Microcosm psionic power, which allows an illithid to take over a victim's mind and make them think they have been transported to a paradise-like fantasy world. In fact, they're just standing around not doing anything. The power negates all normal sensory input and the victim can only be brought out of it if they can realize that what their senses are experiencing is false.

Order of the Stick 885-891 (3.5e based, 2013)

The Order triggers an epic-level illusion trap that allows them to each have a 'Happy Ending' to their quest. Most of the party share a common illusion, but Belkar is in a separate illusion because his conception of a happy ending is not shared by the others. It is unclear how long they are trapped, but for "at least a few hours" because it has been long enough for Roy to be really hungry but not so long that he needs to shave. Elan manages to break them out of it when he realizes that while the other's happy endings are at least plausible, his is so stupid and childish that it should never happen, and yet it does. Once they are free, Roy tells Elan that he did well, and Elan is so unused to the complement that he suspects they are still in some sort of illusory wish-fulfillment trap.

Dragonlance: Shadow of the Dragon Queen (Fifth Edition, 2022)

According to TV Tropes,

[T]he mirror of reflected pasts paralyzes anyone who looks into its depths by showing them an idealized version of their own past, unless they make their saving throw to resist the effect.

Answer 2

You suspect partially wrongly; you just got done with Stardock from level 16, and since you didn't have a TPK earlier in the dungeon, DM fiat swept you up into the most irresistible fun-for-the-DM plotline on level 17: the psi pods and Alterdeep.

A Visit to Alterdeep

If your party of adventurers is incapacitated or killed in Undermountain, you have an unusual option for keeping both them and the campaign alive: the characters could awaken in an alternate version of the Yawning Portal that is part of a psionically generated simulation of Waterdeep called Alterdeep,

As for other adventures: the module Six Faces of Death contains a fake happy ending trap set by a villain.

The characters are then told that a dimensional portal in the cube can take them home, whereupon they are led to area 6.25. As soon as they pass through the doorway into this area, that doorway disappears behind them.

Everything in the new environment appears to be real. Moreover, the characters are also greeted by up to 1d4 + 1 people or creatures that seem to belong at the new location—all of them manifested by the oblex. If the area would normally have a larger group of people, they are unexpectedly absent or show up later, but no more than two to five other creatures are ever seen at the same time.

In reality, the characters remain in the barracks as transformed by the balhannoth. The skull lord simply wants them to waste time figuring out how to get out of this false environment while the pixelating curse overcomes them.

Among the many things likely to give away the false nature of this homecoming, any object created by the balhannoth is clearly a fake. Books turn out to be filled with blank paper, mechanical devices are crude, nonfunctioning duplicates, gold and jewels turn out to be made of paste or tin, and so forth.