Good afternoon.
The ability (morale must be at least 1 for everyone around) – besides the banshee's scream, there are also artifacts that lower morale, and new dragons for the Necropolis.

Dragons give -1, meaning +1 morale remains (since it's initially +2), and using the banshee's scream when you don't have the spell of terror is only possible due to a lack of powerful spells in the spellbook, which is rare for a Necromancer who already has upgraded dragons. It's much better to cast mass spells of darkness, berserk, enthrall, raise dead, summon phoenix, and many other things, rather than -1 to morale. Therefore, the situation where morale drops below +1 is more exotic than realistic...
And artifacts... can be found by any race, not just the Necropolis, besides, there are more artifacts that raise morale, and they can also be found on your hero. + various objects on the map that increase morale are also present...