Collect Halloween Alphabet Box (tradable) by killing Undead and Demon monsters. Give the alphabets (non-tradable) HALLOWEEN to Event NPC in Reboldoeux for Halloween Candy x5 (Combat EXP +30% for 30 minutes). You can also trade any 10 of the same alphabet for 1 Halloween Candy.
Abyss Ore (costs 100 candies in America, 75 candies in Taiwan) can be traded with Event NPC for Abyss weapon costume, which grants ATK vs Monsters +10% for 30 days (activated upon reception). This is available only for broomstick, lightning bracelet, fire bracelet, special bracelet, book, pistol, rifle, crossbow, large caliber rifle, cannon, dagger, polearm, great sword, gaiters, and tonfa.
Other items include Witch Hat for Sierra (100 candies), Coffin of Jack O Lantern (100 candies), Jack O Lantern Hat for Claire/Ludin/Ramiro/Tiburon/Viki/Cano/Rachel/Montoro/Sorang/Rose/Basic (100 candies), and Gift from Jack O Lantern (20 candies), which gives a random item like Soul Crystal, Pet Food, Principal/Enhancement Ampule, Enchantment Tranquilizer, etc.
I don't know... another AFK event. Considering the current price of Feso (i.e. Pet Food), I'm not sure if it is really worthwhile. It's the same problem with King Crab Event, which requires too much event items - too expensive to buy with Vis and eats too much Pet Food to farm yourself. The reward is not even particularly attractive. Maybe it is their way of replacing Play Time Event (which is an AFK event without consuming Pet Food) with a different AFK event that eats your Feso...
Halloween Event 10/22-11/05, 2014
2014年萬聖節活動 10/22-11/05, 2014
Sierra's Dreadful Halloween 10/28-11/11, 2014
할로윈 축제를 즐겨라 10/30-11/20, 2014
Japan server has a different Halloween event - ルディンのハロウィンTrick♪ .