Genius

Edward's instincts were sharpy, and he watched all Zelhart's mana being drained for one spell.  Feeling anger and fear for his life, this was a trap set by Nature, and he shouldn't have come alone. 

'Is this my death?' He wondered. 

But then, the whisper again. 

"Not yet, this spell releases a powerful wave of dark energy, the first blast is the most powerful one, now do it." 

Whoever was whispering to Edward had seen Abyss Void already, considering the first blast as the moment when it exploded and the second as the moment when the dark energy is released to the environment, killing everything it touches. 

Zelhart fired it, not caring about the people around nor his army, Abyss Void would completely obliterate them from existence, and the undead using a crystal would probably have their energy destabilized, dying forever.