Complex sentence: So his son instead of RaNavghan is brought to king, as no one could differntiate between RaNavghan and Uga, King tested him, by asking him to kill RaNavghan (who was actually Uga) by his own hands.
Split into simpler sentences: So his son instead of Ra Navghan is brought to the king, as no one could differentiate between Ra Navghan and Uga. The King tested him, by asking him to kill Ra Navghan (who was actually Uga) by his own hands.