i also skipped the swamp boss and killed modor
i got the horde event twice and and ground is shaking at least 4 times since
so its just rng that you haven't got it

i don't know what enemy spawn during horde, i assume its goblins, my base is built on some land i raised surrounded by water so no enemy's can actually spawn during the raids, which is cool because i never get bothered but also bad because those enemy's drop useful items which i can't get