The Interdimensional Space Time Continuum Bait-and-Switch Ground Rules

One of the most perillous moments in any D&D campaign is the Interdimensional Space Time Continuum Bait-and-Switch. If you do not know what that is, let me explain:

It is when the players move into a different dimension and/or break in space-time such that things become Different to How They Were.

Two examples from my own life:

a - PCs are in a dungeon where they encounter a place where, due to a gigantic lightning strike, a rift has opened into the quasi-elemental plane of lightning and all sorts of entities from that plane have come pouring out. For reasons best ascribed to momentary insanity, the PCs go through the rift an emerge into the quasi-elemental plane.

b - There is a room in a dungeon which although only a dozen or or so yards in diameter contains, tardis-like, a region of desert dozens of miles wide, where a djinni is held captive in a certain hidden location.

Such moments are dangerous because they shift the tone of the campaign and may frustrate an expectation that has been allowed to build for weeks, months or years that things are a certain way. This may not manifest itself in howls of outrage, but it can feel like a clunky gear change, and it can all of a sudden make for a different 'feel' that results in verissimilitude-breaking incongruity. 

You may be familiar with the old Holodeck episodes of Next Gen, which had that kind of a quality to them. You were sitting down to watch regular Next Gen only to discover that, this week, the gang would be trapped in the Wild West/the 1920s/the 1800s/Sherwood Forest* chasing Moriarty. You had wanted phasers, tricorders, weird aliens and the Riker Maneuver. Instead you were getting silly costumes and an end-of-term, demob happy cast playing dress-up. There was a sense of vague deflation and discombobulation. And this can happen during Interdimensional Space-Time Continuum Bait-and-Switch moments, too.

This does not mean the Interdimensional Space-Time Continuum Bait-and-Switch should be avoided at all costs. There is nothing wrong with a shift in gears if you've been pootling along the M6 in fifth gear, stuck behind a lorry, and the fast lane opens up for you. My ground rules would run as follows:

1. Make sure an entrance to another dimension has a readily available exit. That way the players can explore if they want, but don't have to. ('You accidentally stepped through a doorway into another dimension and there is NO WAY BACK EXCEPT THROUGH SOME OBSCURE OR SECRET METHOD!' is, I think, screwing with the players unless it is the entire premise of the campaign from the outset.) 

2. Do not make the other dimension markedly more dangerous than the current dimension the PCs are in without hints that it will be so.

3. Rifts to other dimensions suck if they could be used to facilitate setting-destroying flows of technology, magic, and so on - again, unless it is the premise of the campaign. You are perhaps familiar with the Chronicles of Amber, specifically (I think, from memory) The Guns of Avalon, in which Corwin uses interdimensional hijinks to bring gunpowder weapons (from memory, assault rifles) into Amber to destroy his enemies by kitting out an army. That type of step-change is fine in a novel, but likely in a D&D campaign to just mess things up too much. 

*Yes, yes, okay, that was a Q episode, but you get my drift.