Originally posted by vinyblaster:

In my current playthrough I killed the first 2 bosses, skipped Bonemass and killed Moder.

Since I killed Moder, I've had the "Foul smell from the swamp" 4 times and 2 or 3 times "The ground is shaking", but I've never had the "Horde" event. I expected this event to trigger after I killed the 4th boss. Maybe it's just a weird RNG.

So you start with a forest animals raid that you might not ever see. Killing Eikthyr removes this event and adds a greyling raid.
Killing trolls adds troll raids, as does killing Elder. Killing Elder removes the greyling raid, and adds the Draugr and Skeleton raid.
Killing Bonemass removes the Draugr and Skeleton raid, and adds Drake raids, blob and oozer raids, rancid and skeleton raids, surtling raids, wolf raids, and bat raids.
Killing Moder removes the Drake raid, and adds the fulling raid. Killing Yagluth removes this raid.
All of these raids can only happen in certain biomes, with Meadows being the most common available location. Currently the safest place is the as of yet unfinished mistlands biome, which can only have the skeleton and bat raids at the moment.
As well, the troll, blob, skeleton, surtling, wolf, and bat raids are all unlocked for the rest of the game once you kill their respective mob or boss.