Complex sentence: The "yazata" s Airyaman and Atar will melt the metal in the hills and mountains, which will flow as lava across the earth and all mankind, both the living and resurrected, will be required to wade through it.
Split into simpler sentences: The "yazata" s Airyaman and Atar will melt the metal in the hills and mountains, and it will flow as lava across the earth. All mankind, both the living and the resurrected dead, will be required to wade through it.