Cellious was a coalition of city-states founded at the beginning of the First Chaotic Period and lasted for perhaps five hundred years. Cellious is located in the present-day region of Ebeta. Among the city-states was lahut, famous as the birthplace of the Order of the Western Wind, the oldest sect of Paladins.
Almost everything known about Cellious comes from records belonging to the Order of the Western Winds, and say very little about the rest of the city-states. Apparently each was ruled by a "mortal king" and protected by a Mage Tower. Lahut's army of paladins were elemental in defending the region against "a great scourge"; however the paladins eventually abandoned the region.
Famed historian Herbet Patric Galactis made a well-reasoned argument that the "great scourge" which destroyed Cellious was armies of undead led by Grey Temple Necromancers. The timing fits during the Golden Age of Necromancy, and necromantic ruins have been found as far north as Rowen. Since Ebeta underwent massive upheavals and geologic activity thousands of years later, no ruins dating to the First Chaotic Period have been found in that area.
Galactis went on to speculate that the various city-states of Cellious turned to necromancy themselves to fight the undead armies, and the Paladins felt this was an affront, and left. The entire region was destroyed not long after. This explains, Galactis said, how the Order could so fully relocate itself, in that the entire Order simply moved someplace less necromantic.