To get this achievement you need to kill EXACTLY 123 people in the USA in 123 days.
So, start your game on casual and pick USA as your starting country. You want to evolve Blood Rage, Lair and Therianthropy. Start by creating a lair in the USA and then use your Blood Rage ability and immediately cancel it by flying to a different country before the kill count goes over 123. You want to get around 80 kills. SAVE YOUR GAME. Go back to the USA and have your vampire slowly kill people until you get 123 kills. Once you get to 123 kills fly to a different country and wait for 123 days to pass.
If you have done everything correctly the news ticker will display Sesame Street Killer on rampage in New York