This gives +4 to morale, -2 to luck, -20% to initiative, -1 to speed, and -20% to the defense and attack of a unit whose morale has dropped.
And for Necropolis, the cost of raising troops is 25% lower, and there's a +5 bonus to attack.
Regarding the neutral creatures, there was a real massacre, even without the Banshee's scream.
The enemy heroes put up a bit of a fight, but not much.
If I had upgraded the Scream of Terror, the game would have become even less interesting.