The behavior of raids seems to have changed quietly over the game's updates a bit.
The current info seems to be that they unlock on the previous biome boss and upgrade when you kill the current biome boss. So, at first a foul smell is skeletons and draghr, and after killing BM you get blobs? This is my best understanding from playing and reading the wiki, I could be off on that.
They also raided my home in progress which was little more than the moat and some rocks and a workbench. Not sure what counts as a 'home' for the raids now; never had them attack one with no fire, walls, or anything before.