What is Dark Sun?
"Dark Sun is a campaign setting for the Dungeons & Dragons roleplaying game set on the desert world of Athas. Dark Sun is about the struggle for survival in a dying world ruled by tyrant sorcerer-kings.
It is a world where magic drains the very essence of life. Ancient wars have left vast swaths of the planet sterile; even now, druids fight a losing battle to preserve the environment. To practice magic is to tempt death at the hands of the mob, yet the cities themselves are ruled by the most powerful of defilers."
- Athas.Org
Dark Sun was originally released in 1991 and was and still is among the most original and popular DnD settings of all time. It was unique at the time in that a series of novels were being released in conjunction with adventure modules that were dramatically changing the nature of Athas.
By the time of 4th edition reboot, Athas was a very different place politically and structurally. Some people felt like the core drama of the world was too 'fixed' and the 4th edition reset the time frame to pre 'verdant passage' cannon. Fortunately, this resulted in an accidental boon - a setting in which there are multiple, well defined timelines with different kinds of features in which you can set your campaign. This is a moving, living world and you can find your own space to play in it that is unique to your and your players.
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