Ao Lu chose for the decisive battle to take place at night; he decided to launch a preemptive attack.
If he did not attack, the human army would kill all the low-level demons. Since the decisive battle was going to come sooner or later, he might as well attack first; he could then choose the best time and place for the attack to take place.
Tens of millions of demons were lying in wait at the deepest parts of the sea, not emitting any noise or aura at all. Of course, many Demon Sovereigns were clear that their movements were being tracked by the formidable martial artists of the human race; it was near impossible to launch a sneak attack.