To use the second move of the Soul Slayer War Arts: Vortex of the Astral Souls, Evan needed a large number of souls and these souls must be captured using the first move of the Soul Slayer War Arts: Vortex of Lost Souls.
The number of souls required to use the second move depended on the level of the souls captured.
The stronger the souls, the fewer he would need.
Previously, Evan had captured a few Rank Three souls using the first move of the Soul Slayer War Arts, but they were not enough for him to unleash the second move. However, after reaching the Eldranthia Empire and encountering the barrier that blocked his path, he knew he would have to use his Shadow Walk skill to sneak in.
Earlier, In preparation to ensure he would have a strong trump card for escaping potential danger after going in, Evan killed many Rank Two and Rank Three Eldritch Horrors, capturing their souls using the First Form of the Soul Slayer War Arts.