A role-playing game published by West End Games in 1990. The full title of the core boxed set was "Torg: Roleplaying the Possibility Wars." The principal designer was Greg Gorden. In Torg, all realities have stores of possibility energy, and each reality has, somewhere, a sentient object called a Darkness Device. This device tends to get itself found by some sort of warlord, who then has the power to become High Lord of their reality, and the necessary knowledge to invade other realities. As Torg begins, several High Lords have discovered a reality - ours - that's unusually rich with energy, and they want it for their own. One of them, known as The Gaunt Man, is High Lord of a particularly vicious and terrifying world called Orrorsh, and he seeks to become Torg, ruler of all realities. The PCs are fighting not just for Earth's freedom, but for the sanctity of all realities and an end to the Possibility Wars.
The fascinating thing about Torg is the way in which it takes the many-have-tried-many-have-died concept of the multigenre RPG system, and actually builds the multiple genres into the story. Even before any of the worldbooks came out, the rulebook laid out the new map of the world: different regions, often entire continents, were given over to different invading realities. Realities advanced or retreated by virtue of the physical eternity shards that its minions placed or its foes destroyed (each reality's shards took a different totemic form - trees for a fantasy world, automatic teller machines for a technological one) - placing a new shard caused a new triangle of territory to switch over to the invading reality. Every Torg worldbook had a map showing reality borders with those triangular edges, and the game's developers at West End reportedly had an enormous world map on their wall, showing the war's progress over the years the game was active.
Torg's starting lineup of invading worlds, or cosms, was only gradually filled out with worldbooks describing each one. When the game was first released, the only worldbook you could get was The Living Land, a Paleolithic world full of lizardmen with a strange religion, which invaded most of North America. The world has fascinating implications, but the book was generally considered to be poorly done, and may have contributed to the game's eventual failure.
But as for the rest of the world: two cyberpunk cosms moved in - Japan was overtaken by Nippon Tech (really more cyber than punk), and France and southern Europe became the Cyberpapacy. A fairly generic Tolkienesque cosm called Aysle took northern Europe - its schtick was that its High Lord had seen the light and decided to try to help Earth if she could. (Her Darkness Device had other plans.) Orrorsh, a sort of Victorian society in which it was impossible to go out at night without being ripped to shreds by a werewolf, took Australia and New Zealand. Upper Africa was invaded by a 1930's pulp-comic cosm called Terra, but the resulting worldbook incorporated the flavor of the invaded territory and was called The Nile Empire. (Nile is considered perhaps the most successful of the Torg worlds and may be headed back into print in some form.)
Two worldbooks were released later: Tharkold, a techno-horror cosm that first tried invading Russia but then moved to LA (to get famous, I suppose), and Space Gods, an advanced spacefaring race that resembled the Aztecs (supposedly because they'd also visited Earth in Aztec times). Another worldbook, though not properly a full-on cosm, was The Land Below, an amalgam of every invading reality's fantasies of what dwells underground. The Land Below eventually kicked the Living Land out of America.
As you can see, the whole meta-game of what cosms did what, to whom, and where, eventually got sort of silly. It was founded on a nifty premise, though: when Torg was first released, it came with a free year's subscription to Infiniverse, a newsletter containing supplementary material and, more importantly, game progress report forms. The theory was that players would mail in the news of how they beat back the Cyberpapacy in their campaign, and if enough campaigns reported that the Cyberpapacy had lost ground through the destruction of eternity shards, then the Cyberpapacy would indeed lose ground - future published game materials would reflect this. This idea was a few years ahead of its time; widespread Net access would have really made it fly. As it was, Infiniverse eventually ran out of steam and was abandoned.
Torg's game system is highly cinematic (meaning that realism is not exactly job number one), built on 20-sided dice (the box came with a neat marbled one) and what amount to luck points. It also involves a deck of custom cards called the Drama Deck. The GM turns over a new card from the deck for every round of combat, and a batch of numbers on the card give temporary bonuses, penalties and effects to the good guys, bad guys, or both. Drama Deck cards can also go into players' hands, and be spent for other effects.
Not everyone likes cinematic game systems, and WEG did some very unfortunate things to the story in their desperation as sales declined. However, Torg ultimately owes its failure to bad marketing. The cover read "Roleplaying the Possibility Wars" and showed you a picture of a barren plain with lightning bolts hitting it. WTF? Even if that did get you to pick up the box and see what was going on, the jargon and some of the early bad writing stood in the way of the game's success. The moral of the story: if you want to pick a cool word on which to base your new game system's whole hook, pick something that doesn't sound as unwieldy and dumb as "possibility."