Blake had cast the deadly spell twice, making sure that not one of the humanoid demon mages would remain alive without a godly amount of luck on their side.
And to say that the myriad of darkness tendril spell was a deadly spell was not an exaggeration as the sole act of casting it had the ability to make the mages that were sensitive to nether freeze in place when they felt the amount of nether that had gathered all around them.
However, Blake didn't just cast a single myriad of darkness tendril spells. He had cast two of them.
So instead of just a split second of shock that they felt, the humanoid demon mages literally feel death embracing them when the absurd amount of nether gathered on their surroundings.