Eren unleashes the colossal titans in the 3 walls which go forth and wipe out 4/5ths of humanity. He is eventually stopped and killed by Mikasa and through magic flashbacks and memory edits done by Eren it turns out the whole thing was a Xanatos/Batman gambit set up by Eren to suicide by cop/Ackerman. This is because there was a magic parasite that was the source of the titans all the time along which could control every single titan except the Attack Titan which was the one Eren got from his father. Eren sees the future because of titan powers and in order to get rid of the magic parasite Eren commits global genocide because he knows his friends will be able to stop him and as a result end the magic parasite and titans forever.

A supplementary issue released later filled in some other details like everyone who had been turned into a colossal titan and sleeping in the walls for years all getting lynched by the survivors once all the titan powers were gone, despite the marching titans being mind controlled by Eren.

Eren unleashes the colossal titans in the 3 walls which go forth and wipe out 4/5ths of humanity. He is eventually stopped and killed by Mikasa and through magic flashbacks and memory edits done by Eren it turns out the whole thing was a Xanatos/Batman gambit set up by Eren to suicide by cop/Ackerman. This is because there was a magic parasite that was the source of the titans all the time along which could control every single titan except the Attack Titan which was the one Eren got from his father. Eren sees the future because of titan powers and order to get rid of the magic parasite Eren commits global genocide because he knows his friends will be able to stop him and as a result end the magic parasite and titans forever.

A supplementary issue released later filled in some other details like everyone who had been turned into a colossal titan and sleeping in the walls for years all getting lynched by the survivors once all the titan powers were gone, despite the marching titans being mind controlled by Eren.