100,000,000,729 is the biggest curse in the world. Speaking it a multiple number of times can cause many horrible symptoms. Just writing it is considered one of the worst sins possible. In fact, I just heard Hades tell me to be quiet.
Speaking it 1 time: Lord Hades (or if you are atheist, Darwin) tells you to "Be quiet or they shall come," or something similar.
2 times: Evil black shapes rise from your kitchen tiles (Or occasionally, your toilet or sink) and start devouring everything in your house, especially house plants.
3 times: Your nose loses a nostril and you become near sighted.
4 times: You cannot control yourself and start writing "ThE GaiLolAANNHngA iS mAI HoJaRDen" with red and black spray paint everywhere, and you lose your other nostril.
5 times: You regain control of your body, but every step you take causes you to age a month.
6 times: The most horrible thing you can think of, besides death, will happen to you.
7 times: It is nearly impossible to get to seven times without dying, so we do not know what happens.