When you first load into the new Lake Foscari map, press pause. You will see a question mark on the map - go to this location.
There will be a coffin surrounded by enemies when approached. Kill the enemies and then step directly on the coffin. You will unlock the first new character, Eleanor. Upon quitting or completing your first run after doing so, the achievement will unlock.
Extra
I recommend using Queen Sigma to make life easier. You may also wait until minute 25 to defeat the boss in this map for another achievement.