It is intended to use Essence.  It uses to be that it lowered unrest by a small amount but was stackable.  It was changed so that it lowers unrest a huge amount and requires essence, but isnt stackable.

i do think bless city is a bit too expensive, but i can see the reasoning. without a prohibitively expensive cost, it might become the Conqueror's Best Friend, allowing you to even-more-easily steamroll your enemies and instantly subjugate captured cities with a single spell