Desolations
The Desolations were the recurring apocalyptic wars waged by Odium against humanity on Roshar throughout the ancient history of Brandon Sanderson's The Stormlight Archive. Across many such Desolations, the world was repeatedly brought to the edge of annihilation before a final abandonment of the defense that had held them in check.
The cycle
Each Desolation was an invasion by the Voidbringers, the spirits and corrupted servants of Odium. Between Desolations, the ten Heralds prepared humanity, teaching them the arts of war, medicine, and statecraft so that survivors might endure the next assault. The Desolations were so destructive that civilization was repeatedly reduced to ruin, with each recovery rebuilding from a shattered foundation.
The Oathpact
The Desolations were governed by the Oathpact, a binding the Heralds undertook to contain Odium. When the Heralds were killed, their souls were drawn to Damnation, where Odium tortured them. So long as even one Herald endured the torment without breaking, the next Desolation was delayed. When the Heralds could bear no more, they returned to Roshar, and a new Desolation began.
Aharietiam, the Last Desolation
After a Desolation remembered as Aharietiam, nine of the ten Heralds chose to abandon the Oathpact and leave Roshar, believing the war finally won. Only Talenel remained bound, bearing the full weight of the torment alone for thousands of years. His eventual return, broken and raving, signals to the world that the Desolations were never truly ended and that Odium's assault is poised to resume.