From the publishers website:
Beyond the domains of the Sorcerer-Kings, danger could lurk in every pool, beneath every rock and in every bush. Only luck can save you from a hungry drake or a raging sandstorm, but to the unwary traveler, death also can come from the smallest insect or from the sweetest flower. Keep one hand on your blade and the other on your waterskin; one eye on the horizon and the other on your surroundings. Then you might live until nightfall. When the twin moons rise, the blistering rays of the sun make way for the numbing cold of the night. Under the shadows, creatures of the night come forth to hunt. They see in the dark what you cannot. Sound and flame will draw them near, while spear and sword may fend them off. Sentient creatures will fight with tooth and claw, mind and spell. Woe to the unprepared trader or escaping slave that crosses paths with the Terrors of Athas.
Terrors of Athas contains:
Over 200 monsters
26 monsters playable as races
New Summon Monster and Summon Nature's Ally spell lists
An extensive list of monsters from other sources that will fit into a Dark Sun campaign