"Popkill" – population killing – is a property of some nations' dominion. Any province under their dominion will lose population each turn; this effect severely reduces their income over time.
Since they have little gold, their dominion also freespawns troops for them to use in their own provinces with their dominion. Since they don't participate in the usual gold economy, these nations play very differently than normal.
These nations are:
Ermor and Lemuria have no national recruits at all, and only use gold to build forts, labs, and temples; the other nations do have national recruits and need to carefully ration the gold they get. These nations generally will dump their income scales in pretender creation. They almost always take Turmoil 3, Sloth 3, Death 3, and Heat/Cold 3, since it is futile trying to get income out of provinces with their dominion and there are other things to do with the points.
MA Asphodel and LA R'lyeh, with less population die-off per candle, sometimes attempt to preserve population by taking a starting dominion strength of 1 so they only have at most one dominion candle in each of their provinces. This results in a much gentler population die-off, allowing them more gold for longer.