“Flight to Salvation” – “Deeds and Paths of Darkness” [4th campaign, 1st mission]

In conclusion – VERY (although it can be omitted, as mentioned above).
Considering the bonuses for reducing morale on Biara, berserker/blinded units and control options are very effective, and they are also perfect for dealing with neutral creatures.
I chose Luck because luck is always useful, especially if in mission 5 you retrain Darkness into Chaos and buy artifacts that provide +50% to earth damage and other types of magic. And with magical luck, artifacts, and high Sorcery... wow!