Complex sentence: When Madoc returns, he joins the Welsh and Hoamen forces, and the battle continues until Madoc kills the Aztec king, Coanocotzin.
Split into simpler sentences: When Madoc returns, he is able to join the Welsh and Hoamen forces that are busy fighting the Aztec forces. The battle continues until Madoc is able to kill the Aztec king, Coanocotzin.