See if the races of living heroes have the Leadership skill, which increases the morale (morale) of the squad. That for the heroes of the undead, you can replace Leadership with Sorrow. Since undead have no morale, Sorrow will reduce morale by -2/4/6 (morale) of the enemy hero's units. This mechanic is implemented in HoM 5.
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I took the info from the wiki HoM fandom
+1 morale for troops of one faction
+1 morale for troops in the city
+1 morale for monster lvl 7 of one faction
+1 morale in battle against undead.
For every +1 morale -> 5% to the chance of "Super Morale". If the morale has been lowered (through Sorrow), this is already a penalty to the characteristics. For undead, "Super Morale" can be replaced with a half health respawn chance? At least that's how I see it.