KoBeWi
The Destroyer
Terraria 1.2 added many variations to the world, giving alternate ores and a biome. It adds a lot of replayability value, but not everyone likes to start few worlds from beginning with hope to see "this new ore". So the idea is to give possibility to get everything in one playthrought (it's purely optional) and do this by... parallel world.
Base Concept
Parallel world is a normal world, but exists as a secondary world to explore, which has everything your world doesn't. So if you have Corruption, the parallel world will have Crimson. If your world is blessed with Cobalt, the second one will have Palladium. Going further, if there were no pyramids in your world, they are guaranteed in parallel world with all the loot you can get from pyramid. And all items you can't get in your world (like you have Floating Islands without balloon, or something) are guaranteed to appear in your second world.
Reaching
Now, how to get to this mythical "parallel world"? Through the portal that is available almost at any stage of game. It's not something you get from start. It requires a bit effort, but it's not like you go for orihalcum with your end-game char, because you had mythril. To open a portal, you need a certain item, called Mysterious Mirror. Piece of furniture/machinery of unknown origin, it will appear somewhere on floating islands.
But it would be closed in special chest, called Mysterious Chest. The trick is that the chest is closed, with no key that opens it. To get the mirror, you need to go to the chest during Blood Moon, which is the only time it can be opened (or any other moon event). That way, time when you can reach the second world depends on your luck/effort, but you can get it quite early.
The mirror is used to craft a Dimensional Disruption Device (D.D.D), which can be placed and interacted with to instantly warp to your mirror world. The recipe for device would be quite cheap, requiring 10 Tin/Copper bars and 25 Iron/Lead bars.
Mirror World
The parallel world you can reach is an ordinary world. It's stored in separate file from your first one (.wld2) and is generated upon reaching it. There are no new stuff there, the only significant thing about this world is that it's generation is based on your "first world", taking opposite values as described above. It's just a complementary world, so you don't have to generate new ones to get something (like when you want Palladium: generate world, defeat WoF, poof - Cobalt, repeat until success). I know it's a source of replayability, but also frustration. And the second world, as stated before, is purely optional. You can just throw out the mirror and fully enjoy your only world.
As for the files, when loading world, only the one you are entering is loaded. The second one loads in background while playing, but it makes sure that it won't cause lags.
And to state it clearly: Mirror World don't have the same layout. It may be completely different, it only guarantees to have stuff the first world misses.
Expanding Idea
If there can be one parallel world, then why not more? This could easily create like post-lunar content, through making completely new world type with all new biomes and hazards and enemies and so. But that's something for another topic...
Base Concept
Parallel world is a normal world, but exists as a secondary world to explore, which has everything your world doesn't. So if you have Corruption, the parallel world will have Crimson. If your world is blessed with Cobalt, the second one will have Palladium. Going further, if there were no pyramids in your world, they are guaranteed in parallel world with all the loot you can get from pyramid. And all items you can't get in your world (like you have Floating Islands without balloon, or something) are guaranteed to appear in your second world.
Reaching
Now, how to get to this mythical "parallel world"? Through the portal that is available almost at any stage of game. It's not something you get from start. It requires a bit effort, but it's not like you go for orihalcum with your end-game char, because you had mythril. To open a portal, you need a certain item, called Mysterious Mirror. Piece of furniture/machinery of unknown origin, it will appear somewhere on floating islands.
But it would be closed in special chest, called Mysterious Chest. The trick is that the chest is closed, with no key that opens it. To get the mirror, you need to go to the chest during Blood Moon, which is the only time it can be opened (or any other moon event). That way, time when you can reach the second world depends on your luck/effort, but you can get it quite early.
The mirror is used to craft a Dimensional Disruption Device (D.D.D), which can be placed and interacted with to instantly warp to your mirror world. The recipe for device would be quite cheap, requiring 10 Tin/Copper bars and 25 Iron/Lead bars.
Mirror World
The parallel world you can reach is an ordinary world. It's stored in separate file from your first one (.wld2) and is generated upon reaching it. There are no new stuff there, the only significant thing about this world is that it's generation is based on your "first world", taking opposite values as described above. It's just a complementary world, so you don't have to generate new ones to get something (like when you want Palladium: generate world, defeat WoF, poof - Cobalt, repeat until success). I know it's a source of replayability, but also frustration. And the second world, as stated before, is purely optional. You can just throw out the mirror and fully enjoy your only world.
As for the files, when loading world, only the one you are entering is loaded. The second one loads in background while playing, but it makes sure that it won't cause lags.
And to state it clearly: Mirror World don't have the same layout. It may be completely different, it only guarantees to have stuff the first world misses.
Expanding Idea
If there can be one parallel world, then why not more? This could easily create like post-lunar content, through making completely new world type with all new biomes and hazards and enemies and so. But that's something for another topic...
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