Chimera Wars

Background
Much of the war's history is unknown. They only thing that is known is that Yami's Chimera was the instigator of the war.

Chimera Rampage arc
Kaminari stated that the chimera he was standing on started the war.

Behind the Scenes

 * Since Yami's age is unknown, its possible that she was the one who started the war and used her chimera in order to accompish this.