Story related. You will earn this on your Bad Karma playthrough.
During the mission "Rail Yard Intel" you will again be presented with a Karma based choice on how to proceed. You can either go with Kuo and get pictures of Bertrand creating the swamp monsters to expose him for what he really is to the people of New Marais (This is the Good Karma choice, or you can go with Nix and help her recruit and bond with the swamp monsters, so she has her own personal army of them (This is the Evil Karma choice).
This trophy will pop after completing the "Nix's New Family." branch of the mission which is the Evil Karma choice.