Do you wish to become like me? Foolish mortal. A White Shadow is cursed to walk in the immortal darkness until the daylight, only to wait until darkness falls again. I warn you, it's a lonely existence. But... I can't deny that it's fun.
Enter a wood of aspen trees. Bring two knives with you, one made of silver and the other of iron. You will also need a living animal. Not like a dog or cat, mind you. Something like a fox or a songbird will work. Falcons work exceptionally well.
Cut your palm with the silver knife and stab it into one of the trees. Repeat these words:
"I have seen the shadows.
Cold, dark and dangerous.
But I am not afraid.
I wish that they would fear me.
I want to fight.
Allow me into your ranks."
If you caged your animal, let it out now. If it runs away, you already failed. But if it stays, you can continue. Hold the blade of the iron knife, handle facing out, as if you were handing it to somebody. Kneel in front of the tree the silver knife is in. Remember, it has to be an aspen tree, or you might attract something else.
No matter the time of day, a White Shadow will appear. Do not look up at it. Stay kneeling and keep your head bowed.
It will say, "The dark is a dangerous place. The Shades are relentless, and they never give up. Do you truly wish to fight them?"
This is your last chance to back out. Say no, and the White Shadow will leave. But if you say yes, she will continue. "Then rise, brethren. A mortal you are no longer."
The iron dagger will start to burn in your hand. Drop it quickly. Be careful now, iron is now your worst weakness. It hurts like hell to touch it. I recommend you start wearing gloves.
Stand up now. the White Shadow will be gone. The animal you brought will now be snow-white. It is your familiar. Take care of it, it will be your only companion. The silver knife will have changed too. Take it, it's your only weapon. With a little practice, you can change it into something like a necklace or a ring, even a sword or gun.
Go home. Go to work or school or whatever it is humans do now. You have a few years before people start to notice.
The real fun starts at nightfall. The first change is the best. Go out and kill something. Maybe you'll save someone's life on that first night. You can't die, but be careful, your familiar can; and you tend to get attached to that furry (or feathery) thing.
As each night passes, however, shit gets real. You'll find real people in real danger. Just wait until your first failure. Not so fun then, huh?
When you get as old as me, you only can look forward to the night. The daylight just doesn't belong to you anymore.
You serve seven-hundred years as a White Shadow. You live with it, you die with it and you pray it doesn't follow you after that.