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In the context of Mystcraft, an Age is a new dimension players can explore, similar to the Overworld, Nether or The End in vanilla Minecraft. Unlike those dimensions, however, Ages have a wide range of possibilities. Players can find themselves in a Skylands age as pictured, or in an endless ocean with giant trees sprouting from the water. Advanced players can even write their own ages.
Travelling Between Ages
To travel to a new Age, players may link by using a Descriptive Book either from a held book or a stand, or by using a Linking Book created in the Age the player wishes to visit. The player will be transported to the Age with their inventory and experience, but the book will not travel with them. If the book was on a stand then it will remain there. If the book was in the player's inventory, it will fall to the ground, open. Books on stands can only take damage from rain and snow, while books on the ground will also take damage from water and anything that can hurt an entity such as a player. Books on the ground also take damage over time, so a covered book stand is the safest way to store a book.
Ages are generated based on the Symbols contained within the Descriptive Book used to link. When using a blank Descriptive Book, symbols will be randomly selected. If a player wants to customize their own age however, they can use a Writing Desk to copy symbols to and from their personal Notebook.
To generate your own custom ages with their own biomes and attributes, you must find your desired Symbol Pages, of which there are many and are generally found in small amounts in Abandoned Libraries in random ages you visit using Descriptive Books
Players must be wary when travelling between ages: in order to get back to where they were before travelling to an age, the player will need a Linking Book created in the exact place they would like to return to. If a player forgets their Linking Book, they will usually be trapped with no way back to where they started. Their only hope is to find a Star Fissure to get back to the Overworld spawn.
If cheat commands are enabled, "/tpx 0" command will return you to the spawn point of the Overworld.
Ages may also be unstable, leading to Decay or other adverse effects. Decaying ages will fall apart around the player, eventually leading to a full collapse. Players must always be wary of shifting earth, as there is no other form of warning. Unstable ages are very difficult if not impossible to stabilize, though Skylands ages are more resilient to Decay. You can learn more about instability in the Stable Ages page.
This is a comprehensive guide that will teach you everything you need to know to successfully use Mystcraft.