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In a battle against a mage or another light hero, light itself won't help, but for example, against another necromancer, orc, or demon (provided you have a ring), a holy word will be much more effective than Berserk.
Your enemy might also have a ring, but there's no defense against Subdue except for counter-spells and antimagic, but then Holy Word won't work either.
You can inflict much more damage with Subdue, especially on a strong enemy unit, than with Holy Word.
