This world is one only for the rich and powerful, life force is everything. Life force is your life span, you can extend it by sacrificing someone else's . Everyone is given a tattoo on their body, a number, this number tells you how many years you have. It goes down one on your birthday. But not all the time people's go down, there's a one in one-billion chance that your number would not go down after some point in time. This all happens after birth, no one has ever been born with this. If you were given this gift, then you stop aging at the age you get it, normally once you become a full-fledged adult. Barely any are before that. You also become immortal, the only way you can die, is if someone stabs you in the head or heart.