So this one comes right out the The House of Hades. After Annabeth and Percy enter the realm of Nyx and convince her to let them take a picture for the tour (by the gods that sounds dumb, but lets keep going), she calls her children, all her children. Nyx's children have caused fear and death for all of mankind and all their horrors are considered evil...but who is the darkest?
Geras-The goddess of old age. This goddess is the one that cases all people to eventually wither and die. She is the reason that you can't run as fast, think as clearly, and have so many more wrinkles then you did when you were younger. Her powers will eventually lead you to your ultimate end.
Eris-Goddess of chaos. She managed to start one of the worst wars in history with a single apple. With a word, she could cause an entire room of people to go mad and argue about complete nonsense. Her presence could bring war on par with that of Ares.
Hypnos-The god of sleep...he makes people sleep...I got nothing. Ok, let me play this up some. He could make you sleep forever, suffering eternal nightmares in a dreamland you will never wake from. He was even strong enough to cast Zeus into the realm of sleep.
Arai-Creatures that bring to life curses placed on you. If you don't kill them, they will attack and eventually kill you. If you do attack them, they bring to life a curse that someone has wished on you. This could be anything from an itchy eye to a massive heart attack. They will also taunt you no matter which option you pick.
I was going to include Thanatos, but he was just in a fight a few weeks back. So which one of these haunts your dreams and cause the most darkness to fill our world?
You can vote for your favorite here.