The national epic centered on a heroic figure on whose actions depended the facts of a tribe.

划线部分为定语从句，修饰a heroic figure. 从句用了完全倒装结构，即主语后移到了句末。（从句谓语动词应该为过去时，depends改为depended）。如果不倒装，则从句为：

The national epic centered on a heroic figure on whose actions the facts of a tribe depended.