The Aetgardian Plague (also called The Great Plague or simply The Plague) was a massive outbreak that wiped out the majority of humanity in the Aetgard region of Arad. Records indicate that the Plague likely began in northern Aetgard and quickly spread southward. Its spread was extremely quick, and no cure was ever discovered.
(Not to be confused with resen infestation.)
The highest concentration of Plague survivors were the people of Lapis, on the southernmost border of Aetgard. When word reached King Diederich that the Plague had struck northern Aetgard, he immediately sealed the borders of his lands, set up quarantined districts and layers within, refused any form of aid or contact with anyone outside, and mercilessly banished any citizen showing even the slightest signs of any illness. While the duchy's population dwindled significantly, Lapis was the only Aetgardian territory to survive the Plague somewhat intact.
The Giganti of Sirak Drauth were virtually unaffected by the Plague, as they had always been wary of their Aetgardian neighbors, and easily sealed the borders to their mountain realm when reports reached them of the disease in the neighboring land. There are, however, tales of some Giganti who contracted the disease and opted to exile themselves and die in order to spare their nation. Records tell of at least one "plague ship" that sailed out into the most dangerous known region of the Great Seas.
When the eastern nations of Arad learned of the Plague, the nations of Khaldea, Nuum, and Ivial agreed to implement and enforce the Quarantine, cutting off the west completely in order to prevent the Plague spreading eastward and across the entire continent. The Quarantine Wall was built in southwest Khaldea, cutting off the Hillfolk (and also providing the sole known incident of cooperation between Khaldea and Nuum). Plans were made to construct a similar wall in the mountain pass between Aetgard and northwestern Khaldea, but a massive glacier was discovered blocking the pass on the northwestern side, often deemed a miracle and blessing from the Immortals to protect the East.