In order to obtain this trophy, you are required to feed the "Queen" three times. You can only do this once you have killed the final boss of the Assault on Dragon Keep DLC story. Once you have done that, you are prompted to feed the "Queen" for story purposes, with eridium. If you have enough (you should, the final boss drops quite a bit of it), you can feed her twice more here and get the trophy. If you for some reason don't have the eridium required, you can do so in Flamerock Refuge, near where Roland stands.
You feed the "Queen" by pressing Triangle while having your crosshairs on her.