Everything grows from the soil up, but Terrin is stuck in the mud.
The other earth shifters create beautiful things, like exotic flowers, trees taller than mountains, while Terrin sits in the manure pit making sure the soil isn't too acidic for cherry blossoms.
He hates it. He truly hates his life. After nearly three centuries of doing the exact same thing every day, he's willing to do anything just to escape it.
So when the goddess's kingdom is shaken by an earthquake, and the garden wall crumbles, Terrin takes his chance. Even though touching the ground of the outside world could very well kill him or at least change him.