You need to pay Priests (instead of Workers) when transforming Terrain spaces. Pay 1 Priest and get 2 Victory points for each step of transformation.
After building the Stronghold, you may immediately and only once trade up to 3 Workers for 1 Priest each (Ordination of Priests).
The Darklings' faction board
Darklings are said to be wicked and smarmy creatures that lure others into dangerous marshy hollows for fun, only to rob and then drown them. Only few have tried to verify these rumors. Darklings are fairly skilled in the elemental cults – that's for sure. Their priests use their secret knowledge to bog even more land.
By Kay-Viktor Stegemann