Cairn of Shadows

From the introduction:

Nearly 10,000 years ago, during the nearly forgotten time scholars call the Age of Darkness, nomadic tribes wandered the land now known as Nidal. These proud people revered and relied on their steeds, and so the other peoples of the time knew them as “horselords.” In this distant age, these people suffered greatly. After the near-apocalypse of Earthfall, dust and ash choked out the sun, plants withered and died, and herd beasts starved. In desperation the horselord tribes called out to the darkness—for food, for protection, for salvation. The darkness heard their cries, and sensed their desperation, and the Midnight Lord, Zon-Kuthon, answered.