"Necropolis" - changes to creatures of levels 6-7.

The damage to 1 adjacent target should be 1.5-2 times greater than the standard unit attack against an enemy with average defense (12). This is if you want a powerful ability.
If you want a normal one, then it is enough to make the damage = 100%, in this case, the advantage will come from the retaliatory strike, and, perhaps, the ability will affect someone else besides 1 target.

Also, it should be taken into account that if the opponent is afraid of the scream, which is only performed from a stationary position and only when there are enemies around, then in fact, such Banshees will be able to scream even when they are half-dead, if they even survive to scream.