Regdar bashed open the partially ajar door and peered into the room. "Twenty feet square. Empty. Four doors," he grunted back to Mialee. "Nothing special. Regdar finds it boring."
"Let's go, then." Lidda slipped past the burly fighter and headed into the room, and her companions followed. As the door swung shut behind Jozan, though, everything changed. Doors shifted to the center of walls, trapdoors appeared on ceiling and floor, and piles of dry bones that hadn't previously been there suddenly appeared. Worse, some of those bones lay stuck to the walls and ceiling. . . and the massive two-headed troll that was obviously responsible for them also appeared, standing easily on the ceiling.
"We've just teleported!" shouted Mialee in a fluting voice. "Or plane shifted. I'm not sure."
"Later," growled Regdar as he looked up at the troll just out of his reach. "We have other problems."
In the recent "what are your favorite classic Dragon articles?" thread started by Erik Mona, I mentioned that I got a tremendous amount of mileage out of the tesseract articles back in Best of Dragon 1. It occurred to me that most folks nowadays don't remember or haven't read these, so I thought I'd explain the idea behind them. The last time I used them, it made for an amazingly fun session.
Tesseracts and hypercubes have all kinds of cool mathematical ramifications that don't matter a darn to your D&D game. Don't worry about them. In the D&D sense, a tesseract is effectively a closed dungeon set as a demiplane. Until it is sealed or the trap triggered, it's just one room. Once the demiplane is sealed, however, the room becomes a sealed plane of eight rooms. Each leads into the next in a never-ending loop, and there is no obvious way out. Even better, each wall and ceiling has its own gravity, making for some fun battles.
So what is it? Picture a square cardboard box. Unfold it, and you have six different 2-D panels that look like this:
Now, what if those panels were 3d instead of 2d? If that were the case, then you'd be looking at a top-down dungeon map instead of an unfolded box. Each "panel" of the unfolded box would be a regular room. In addition, there would be a 7th and an 8th "panel" that would sit above and below panel A. We'll call those G and H, set those aside for a moment, and come back to them in a bit.
A dungeon tesseract is typically entered through a trapped room that seems normal while its entry door is opened. It can be placed into any dungeon, and is typically 10' x 10', 15' x 15', or 20' x 20' (depending on the type of tactical combat you want to have occur; more on this in a minute.) Once all four doors of the room are sealed, however, everyone within it is thrust with no saving throw or SR into the closed demiplane of the tesseract.