each boss kill both opens up a new base raid type and closes the previous raid type. Then there are regular raids not associated with boss killing that will randomly happen at whatever base you happen to be at. The worst raid types are from the 4th and final bosses. Raids have a chance to happen after the 9th in game day.

Clearing monster spawners and troll caves will reduce the static monsters in an area.

each boss trophy hung will also introduce basic mobs from that bosses biome to all lower level biomes. For example, once you kill the final boss then Fuling squads will start appearing in the Meadows.