The Landpoison of Yamato was a disease which had slowly ravaged Yamato through the Cursed Earth in the Third Age. It had originated from the corruption of Yggdrasil, the first World Tree, on the Isle of Yggdrasil in the First Age.
The corruption of the World Tree had slowly spread from the isle to the land mass that became known as Yamato which had been most affected by Mardük's taint due to the God of Chaos being imprisoned there. As a result the poison had turned the land of Yamato more and more inhospitable so that by 1000 AE it had driven Yamatians desperate enough to seek fertile lands elsewhere, which led to the Unification of Yamato and the subsequent Yamatian Invasion of other nations.
Leon Alcibiates led Geraud Aurelac and the Silver Branch to the Isle of Yggdrasil during the Great War and found the withering World Tree. In order to heal the Tree before it would die, Geraud sacrificed himself by fusing with the Silver Branch and Yggdrasil's remaining energy to become the second World Tree, the Silverbranch Tree.
The Isle of Yggdrasil magically drifted into the middle of Yamato during the great land shift caused by the Cataclysm, and the new World Tree was rooted in a well-protected area deep in Yamatian territory. The uncorrupted Silverbranch Tree managed to purge Yamato of the Landpoison soon after Mardük had perished and the Cataclysm had ended, thus strengthening magic in the world and making Yamato fertile again although by that point the demons had invaded and enslaved the Yamatian people in this new, verdant land.