Hanji had developed a poison from Annie's diamond fragments . We had found out their secret of reproduction it was a cave whenever one died one was born , however the titan shifters were a rare breed as they were artificially made by being injected with a serum , like MY Eren was . On the contrary since Grisha , the only one who knew about the serum , is dead we don't have to worry about any more popping up. The plan was to attack the Titans in the cave, spray the gas-like poison and watch as it spread. I don't own any characters , pictures or places . All rights go to original owners