Cyberdimension Neptunia: 4 Goddesses Online (EP)

Depending on your playstyle and how thoroughly you clear dungeons of monsters, this could unlock at any number of times but will most likely pop while finishing off the guild quests (for me, it unlocked during the Rank 6 missions). To see how many monsters you've slain, go to the menu and click "various records" to see how many you've defeated.