Netheril

Netheril, or the Empire of Magic, was a historic human empire in the Forgotten Realms campaign setting. Aside from the empire of High Imaskar, it is held by scholars of the modern Realms as an example of the heights that Mankind might reach, as well as how hubris can bring down even the mightiest of empires.

Refugees from the fall of the empire due to Karsus' folly eventually travelled south and founded the state of Halruaa, while the enclaves of Shade (who worshipped Shar) and Opus (who worshipped Selune) were traveling the planes when the disaster struck.

Seriously, if one thought that the Realms had ridiculous power scaling as far as wizards went, read the sourcebook Netheril: Empire of Magic. On average, the NPCs presented are in the mid-20s in levels, and Karsus himself weighed in at level 41. That's how insane it is.

Geography

Netheril encompassed what would now is the Anauroch desert, and was split between "High" and "Low" Netheril. High Netheril included the snooty magic-users, their followers, and everyone else who could afford to live on the Enclaves -- flying city-states all led by their most powerful Arcanists. There were at least 54 named Enclaves, and were considered centers of learning and enlightment, as far as snooty magocracies go.

Low Netheril was made up of the small towns and villages called Demesnes led by minor Arcanists who had not yet level-grinded enough experience points to lift their holdings up into the skies. Commoners weren't really given much respect, so its no wonder many didn't have a good opinion on wizards, and instead turned to the Gods for comfort.

Karsus' Folly

As previously mentioned, Netheril is known for many things, but mostly due to how it ended catastrophically, all because one dude thought he could do better than the reigning Goddess of Magic. Said dude was named Karsus, and is historically known as the only wizard to ever cast a level 12 spell. No Epic-level spell slot BS, this was a true leveled spell, with all the power that entailed.

The spell was called Karsus' Avatar, which should tell you a lot about his ego, but it surprisingly worked. For a singular moment, Karsus wrested the control over the Weave from Mystryl, became a Greater God, and with all that power came the knowledge that he had monumentally screwed up -- he was not up to the task of managing the Weave. At all.

Mystryl sacrificed herself to stabilize Karsus' colossal fsk-up, being reborn into the Goddess Mystra, and the latter reconfigured the Weave with rules to prevent shenanigans like this from happening ever again. Magic across Faerun failed, and because of this all those fancy flying Enclaves came crashing down to the ground.

As for Karsus himself, he was killed when the magic disappeared, and his mortal form was turned into a massive red stone that crashed into the High Forest. The last thing he saw before he died was his entire civilization being destroyed because of him. Well done.

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  Netherese fighting their greatest foe, a flying tube sock