Torax is a giant rock monster who can be found randomly on the game map - he does not appear on all maps. No quest needs to be started to spawn him. He is found in a mountainous area, marked as being monster territory named the Lombard Desert. The game will give you a notification about the lord who levels mountains and add a quest when you stumble upon his area. An Epic level army is recommended to take him on.
If you don't acquire this during your process of grinding out win achievements, you can use a custom faction with the Master Scouting and Stealth faction traits, and scout spam random maps until you find one that has it. Make sure you set the map to have a high occurrence of wildlands to increase the chance of the Lombard Desert being included.