The attack that the alpha used is simple enough to understand. Basically, there is a way for it to follow the intended target. This explains why the attack is always present no matter where Asther chooses to shadow dive.
This is one of the reasons why he decided not to answer the alpha a third time. He used this as an opportunity to understand the spell that the alpha used on him. The necromancer doesn’t want the fight to drag out with his opponent showing one move after the other that he doesn’t know how to deal with after all. That is why he reduced the number of the possible kinds of spells by doing what he just did.
He is also able to figure out that, the attack would stop when it makes contact with something that is “alive”. Asther knows this is a fact because, even when the blades
