Dude... Then 2*(10 + morale (because minus minus equals plus))
Math!
Added 9 minutes later
It increases. It will be 2 * 11, etc.
For example, 217 banshees deal 4774 damage to a target with -1 defense with their ability, which is equal to 217 * 22.
Added 5 minutes later
However, this is a weak ability because even with a low hero attack, the banshee's damage is significantly reduced compared to a normal attack. Unless the enemy has -99 defense. So, it slightly weakens the enemy if there is no opportunity or desire to simply attack, kill units, phantoms, etc.