It turns out at the end of Season 3 that the monsters they fought for three seasons were actually exiled members of (minority - same as the main character ethnic group) in a place outside of the previous main setting, which had a curse that could turn them into giant mindless carnivorous monsters and were dumped on the island on purpose. Plot happens and they come into contact with the civilizations that did this, and they're hostile. Tensions escalate and Eren, the main character, decides to use a secret weapon and start the "Rumbling", which is basically mass genocide of everyone who does not live on the island with them. The other main characters try to and eventually do stop him, but enough damage has been done. In the epilogue the society is a militaristic absolute monarchy prepared for forever war against invaders and the main characters bizarrely thank Eren for having done the genocide for them, even if it was incomplete.