I hate this quest. It has been in since 2010. I remember doing this when it came out....
Start in Prontera.
Here we find Lina. She asks for our help and to take a sacred oath.
Now we must kill 50 Deviruchi, Wraith Dead, Dullahan, or Nightmare Terror. Your best bet is in Niflheim, go to Eden Kitchen, talk to Bathory.
Now we gotta go to 4 different locations and talk to the dark manifestations:
Return to Lina and she will reward you with Exp and an Easter Egg Shell Hat.
I hate this quest. 0/10 would not recommend.