I defeated Yagluth a long time ago but I still get fulings. It seems to me that it was not until the defeat of Yagluth that fulings appeared in other biomes for the first time.

Our character is not ready for fulings immediately after defeating Moder. Moreover, the justification for events being enabled by the defeat of a boss seems to be that some discipline the boss was exerting on its troops vanishes. Maybe they go rogue, or maybe the next boss is able to recruit them.

However, in the test branch, after the Mistlands update, I started to get "They sought you out events" not long after defeating Yaggs, so it's more complicated that that. In the first one, seeker soldiers smashed through my curtain wall before I was able to kite the action away. It was a near disaster, and my idea of a good time.