World fusion (世界の融合, Sekai no Yūgō) is a key concept in both Suikoden Tierkreis and Genso Suikoden Tierkreis Hoshikuzu no Shiro. It is the merging of a fragment of one world of the Infinity into another world.
From the perspective of this world, world fusion is when a part of another world appears in their own. A region from another world, such as a mountain range or a forest, could appear along with the people and buildings in that region, changing the terrain, environment, and climate of this world greatly.
Additionally, everything that existed in the spot where the new region appeared will disappear and also be erased from people's memories. People from both worlds will also have their memories changed to believe that the world in its current state is as it always was. This is through the power of the True Chronicle.
When world fusions occur is dictated by the True Chronicle, and it is possible to artificially induce world fusion by rewriting the True Chronicle with a False Chronicle. Valfred, for example, used the True Chronicle to determine the timing of world fusion and then used his collected False Chronicles to instigate them in useful locations and times.
Two notably destructive world fusions were the 9th world fusion, when the appearance of the Marsinah Plains wiped out the western half of the Ch'olui Mountains as well as some northern islands of the Lonomakua Chiefdom, and the 10th world fusion, where the appearance of the Janam Desert completely erased the Magedom of Janam.
- Genso Suikoden Kiwami Encyclopedia, page 740