Some supplements suggest that these realms are all alternate universes of each other, and someone from, for example, Eberron could reach Athas by travelling through magic portals. In the Dungeon Master's Guide 2, the desingers go into detail about a dimension known as Sigil, the city of doors. Sigil connects many realms in the D&D universe, and can be used as a way for characters from a world of high fantasy to go on a steampunk adventure for a change of pace. After they do so, they usually return to their own universe and all is well. Because a person could potentially reach any place in any universe through Sigil, it is strategically the best spot for an evil ruler to want to take over. The god Vecna once tried to usurp the Lady of Pain, who is in charge of Sigil, so he could take over the entire multiverse through the many portals in that city. He failed, but that didn't stop others from trying.
One fateful day, a wizard, whose name was never known, from Abeir-Toril came to Sigil. He had been in Toril during the Spellplague that practically tore apart the realm, and had watched the world rebuild. He had stayed in his broken tower to study the effects the Spellplague had on the world, and from there how he could harness its power to use to help the world. He soon realized that the collapse of the Weave did not create the plague; it opened up a rip into a parallel world that was alike his own in every way, except for the way magic was used. In this other realm, magic was corrupt and vile, and some of that magic escaped through the rip when the Weave was ruined into the Abeir-Toril that this wizard knew.
It got the wizard thinking. If there is another universe with only slight differences between his own, are there others? Does every possibility create a new universe in which a different outcome happens than in the other universes? Further study revealed this to be exactly the case. And the one place that all those universes were connected to was none other than the city of Sigil. The wizard had begun his research to try and cure his land of the Spellplague, but anger at the events of the universe corrupted him. He didn't want to help others now; he just wanted to undo the pain of his own life and live in ignorant bliss. He spent eons researching a spell that would allow him to find another dimension that was like the one he loved but without the destruction that tore it apart. As soon as he discovered it, he rushed to Sigil to cast it and be relieved of his current life.
He was too rash. The spell was not complete; it would connect him with other universes, yes, but it would not separate itself from the effects of his parallel selves casting the same spell, casting a slightly better spell, casting a slightly worse spell, casting a much worse spell, all at the same time. The result was catastrophe.
Every possible universe, from Abeir-Toril to Eberron to Athas and all their reflections, were forced together into one through the spell cast in Sigil. Everyone from the lowliest kobold to the mightiest god reeled when every reflection of them was forced into the same dimensional space. The wizard tried to call off the spell, managing to stop the universe from replicating within itself until it imploded, but the damage was done. No one's universe was the same anymore, and they all now had tears and holes into each other, cracks that no spell could fix.
Where castles stood in one universe and were crumbled in another, there now were half constructed messes of stone and wood. Whereas gods watched over the world in one universe and were disconnected in another, they now had contradictory memories and altered abilities that resulted in even more destruction when they tried to fix what had been done. Mortals, even those who were not yet born, were thrust into a constant state of change, fluctuating through each possible parallel of themselves. Beings that had been locked away in most universes were free in others, and now were able to further the goals that others had deeemed too dangerous to let them continue.
This new universe is one of every possible fantasy trope, even those that are contradictory being in the same individual. Heroes used to make a difference in their world; now they're just lucky if they survive the unpredictability of the terrain, the monsters, and even their own selves. No one has given a name to this new universe, this conglomeration of fantasy. One scholar tried finding out the name of every universe that had been forced together so he could name the new universe after a mixture of them all, but stopped after he realized the name would be longer than Gredarxygnevcatebechrjunelffinpeoforkon. The only name that really gives it justice is Dungeon World.
Dungeon World is based on the D&D Gamma World game, but using fantasy instead of science fiction. Dungeons and Dragons lets you tell fantastic stories about heroes and adventure; Dungeon World lets you bring random silliness to your game just for kicks. Dungeon World uses the same mechanics as Gamma World with a few nominal differences.
- Instead of choosing two origins, you (usually) choose a race and a class. This can be done randomly, but you're less of a big chicken if you decide not to than you would be in Gamma World.
- The ten skills and the ability scores they use are as follows: Acrobatics (Dexterity), Arcana (Intelligence), Athletics (Strength), History (Intelligence), Insight (Wisdom), Interaction (Charisma), Nature (Wisdom), Perception (Wisdom), Religion (Intelligence), and Stealth (Dexterity). Arcana can apply to technology, and dungeoneering is rolled up into Nature.
- Each player has a deck of talents instead of Alpha Mutations. These usually have prerequisites specific to their character. If they experience spell flux (the fantasy term for Alpha flux), they must replace their talent with a mutation from the DM's deck, which does not have any prerequisites. Only after the battle can they draw another talent.
- Magic items are handled exactly like Omega tech. Magic is very unstable in Dungeon World, and magic items are prone to fizzling out after being used too much, though a skilled adventurer could salvage it to make something still useful.
- Guns do exist, though usually in the most primitive sense. The gun mechanic can be applied to common magic implements, which can be used by anyone to blast out a simple magical spell, but if used too much in too short a time will need time to recharge (or for guns, get more ammunition).
- Other gear is also more fantasy-like. I'll come up with a table eventually, because your starting gear is still determined randomly.
What do you think of a world that combines the tropes of fantasy with the themes of science fiction? What would you like to see mechanics for that the rules of normal D&D 4e didn't allow, but it could be possible with Gamma World mechanics?