It seems like I've been telling
everyone HOW...
Why not post it on the message board as well?
The Dark World system works like this.
You make a map as you normally would. Then move the lower left corner of the map about 64 fracunits away from the "origin" point (0,0) which is highlighted by most map editors (But not Doom Builder, for whatever reason.) ... The origin is the only point on the map where the light world and dark world actually "touch", so if you want to do any fancey room or hallway in the middle of the two worlds, that's where to do it.
Then make a copy of the entire map and rotate it 180 degrees and place it on the exact opposite side of the origin so that it is a mirror of the original map, as you would if you were making a CTF map, except the maps are both seperate of eachother.
Then make any changes to the second map you would like, as this is your "dark world". You may change textures, Thing positions, move and alter whole sectors, whatever you like.
Then create a two-way linedef executor teleporter which moves you between the two worlds. I'd create a special addition to the teleporter linedef executor that rotates you 180 degrees, thus saving your direction when you entered and such, among other things, but there isn't much demand for it at the moment. :\
You may also wish to use a music-changer, as happy light world music wouldn't go well in teh dark world, and even a sky changer so you can have an evil sky.
Finally, make a map header and add "darkworld = 1" to it as you would any other map option. I would add "darkworld = 2" to have 3 worlds or "darkworld = 3" to have 4, but again, there is no demand for it. :\ Heh, with "darkworld = 3" somebody could make different worlds in which one has fire, another has water, another has large empty holes ripped in it and death pits, and another has grass all over. Or whatever. It'd be cool. ^_^
And if you want to get REALLY advanced and have only one person in the dark world at a time, you can make it so executing the teleporter changes its tag to point to another linedef executor that tells people that someone else is in the dark world so they cannot enter, and all the teleporters back to the light world tag the teleporter to the dark world back to the way it was.
All this and the only thing I did was add "shadows" to tell you where people are who are in the other world... Heh...