When casting the World Sword, Lin Li had already prepared his backup solution out of a habit that he had developed through experiencing countless battles in the Barbarous Arena. In other words, he would have numerous sets of attacks that he would cast consecutively, and never let himself think that he could defeat his opponent with just one move.
From the performance, it seemed that the purifying power of Holy Light should be the nemesis of the vicious and filthy power. However, without the Stars of Fury, Lin Li could not utilize the power of Holy Light to fight the power of the prehistoric Viper. Hence, he thought of Gloomy Dark. As the saying went, "if you can't beat him, join him". Becoming one with it was definitely easier than destroying it.
