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A Sacred unit is able to be
Blessed.
Aside from that, Sacred units require half as much Upkeep, but they typically cost more to hire than non-sacred units of the same skill set. They're also vulnerable to certain things:
Last-but-not-least, sacred units require Holy Points to hire. Every turn needed to recruit them (from not having enough Commander Points or Resources when you recruit them), they require a Holy Point, though some require two. Your total amount of Holy Points for each fort is equal to your Pretender God's Dominion strength, plus 1 for every five temples you have; or, for Ind and Piconye, one-for-one.