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In short. Morale is good when you need to defeat two enemies, and luck is good for defeating one without taking damage.))))
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Honestly, in such cases, I take Luck (both when playing as Necropolis and as Swamp – I usually play as them).
I don't quite remember if there's an artifact called "Spirit of Despair," but if I remember correctly, it removes all bonuses to defense... (Please confirm if this is true). It's often found on maps, and it's also needed to assemble the Armor of the Elements. So, I think Luck is preferable, especially for ranged units, because elves have two shots, and if they have Luck, it's amazing.
In general, I try to avoid these skills; I think magical skills can be put in their place (because I usually play as warriors), like Wisdom or Earth/Air Magic.
Yes, this "Spirit of Despair" removes bonuses to leadership and luck.
Leadership is only better for certain castles, as it doesn't suit some races.
The Spirit of Despair only reduces positive morale to zero. It doesn't raise negative morale to zero, so it's useless with a bunch of level 7 units from different castles.