This familiar crack was the hope Igor staked everything on. Sometimes, gambling is a last resort, a desperate measure when one is pushed to the brink, and that was precisely Igor's current state. He knew that if his gamble went wrong, the consequences would be severe. At that point, no one would be able to help them; they could only rely on themselves to survive.
Imagine if Igor's entire conjecture was wrong: this Dark Force would not fear any of their attacks, as all their attacks would only be aimed at the mere vessel that housed him. Attacking the vessel would be pointless, as the Dark Force could simply find another one. Consider the Dark Force being in a state of invincibility, where no one could defeat him.
