The grand migration.

Population Migration / Travel

Migration from their previous home to the only hope they had.

The grand migration was the life saving decision to leave behind their slowly collapsing homes and move to less comfortable, yet much safer Atocites.
  The event itself spanned whole month, as the migration happened at several different places, to numerous different Atocites. And whoever was left at The Ground bellow after the migration was done was lost either to the monsters or to Corruption.   The evacuation of entire civillisation like this was possible only thanks to people who specialized at monitoring the environment. The ancestors of the first Scouts. And people who decided to evaluate every possibility before taking actions. The ancestors of the Planners.   At the time of migration, the Atocites were still fresh, and had a lot of ground and stones that either collided with some of the higher hills or were in reach of dedicated buildings. People used the cycles in which the Atocites fly to prepare for the migration.
When the time came, the strongest quickly climbed and helped the others move as much as possible.
But it quickly became apparent that to survive on their new homes, they had to periodically return to the quickly evolving ground bellow.

For the purpose of securing necessary resources. The Raiders were trained.