Every time you kill a boss, it unlocks the base attacks/Raids for the next boss. Basically whoever your next boss fight is will send enemies to attack your base, until you kill the boss of that biome.

When you first start the game and haven't killed any bosses, Eikthyr will raid:
"Eikthyr rallies the creatures of the forest"

When you kill Eikthyr, The Elder will start sending raids:
"The forest is moving..."

When you kill The Elder, Bonemass will start sending raids:
"A foul smell from the swamp"

When you kill Bonemass, Moder will start sending raids:
"A cold wind blows from the mountains"

When you kill Moder, Yagluth will start sending raids:
"The horde is attacking"

Each of these raids are affected by enemies you kill as well, sometimes adding them to the raids (like Blobs to the Foul Smell From The Swamp raid) and also killing bosses will permanently add raids as well, such as "The ground is shaking" Troll Attack which happens after you kill The Elder, and have killed a Troll.