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The chance of recruitment depends on: Race, Hero, Money, Army, Mood of neutral creatures, and, of course, the Diplomacy skill. Someone calculated it using Ossir (a Sylvan hero specializing in archers) as an example. If he invests in Diplomacy and keeps Archers and Hunters (upgraded archers) in his army, the approximate chance of neutral elven archers joining him is around 50%. I don't know how they calculated it, though. That's how it works.
