Dead Man's Party

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• Catrina Location:
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She is located in graveyards outside of every major city, both Horde and Alliance!
She will be standing there in the form of an undead, dancing.
To get the achievement, simply do /dance on her!
You will also get a nice 12 hour buff named "Honor the Dead" which is simply you honoring the dead!

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• Quest:
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If you use either a Bouquet of Orange Marigolds or an Orange Marigold (bought from the vendor there) you will be able to see a bunch of spirits. One of them is a quest giver and will give you a quest named "The Grateful Dead". Long story short, they want you to give them some Bread of the Dead. When you complete the quest you will be awarded with either exp or money depending on your level and a Macabre Marionette which is some kind of pet which requires the Day of the Dead. This probably means that it's not usable after the Day of the Dead.

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• Vendor:
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There will also be another NPC named Chapman who is the <Day of the Dead Vendor> standing there selling the items:

Candy Skull

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• Requires Day of the Dead
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• Use: Restores 61 health over 18 secs. Must remain seated while eating.
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Orange Marigold

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• Requires Day of the Dead
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• Detect lesser invisibility for 30 sec. Only works during the Day of the Dead. (30 sec cooldown)
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• "Said to attract spirits and help one communicate with the departed on the Day of the Dead."
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Spirit Candle

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• Requires Day of the Dead
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• Place a Spirit Candle that increases the spirits of those nearby by 4. (5 min cooldown)
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Recipe: Bread of the Dead

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• Binds when picked up
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• Duration: 2 days
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• Requires Cooking (1)
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• Use: Teaches you how to cook Bread of the Dead

Bread of the Dead:

Requires day of the dead
Use: Restores 2 % of your health per second for 24 sec. Must remain seated while eating. If you spend at least 10 seconds eating you will become well fed and gain stamina and spirit for 15 min.
Requires Simple Flour, Ice Cold Milk

Whimsical Skull Mask

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• Binds when picked up
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• Head
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• Requires Day of the Dead
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• Item level 1
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Bouquet of Orange Marigolds

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• Binds when picked up
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• Held in Off-hand
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• Item level 30
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• Use: Detect lesser invisibility for 30 sec. Only works during the Day of the Dead. (30 Sec Cooldown)
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• "Said to attract spirits and help one communicate with the departed on the Day of the Dead."
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Ugghhh, and nothing of that is wrong since I personally wrote it in detail straight off of the vendor himself in-game! That took some time! xD

Why isn't the vendor or the items he sells listed on wowhead? Is everyone so busy with 3.3 patch that world events get lost in the shuffle? Sad really.

For those of you like me that don't have the locations of all the starting graveyards memorized, here is the list:

Horde

Silvermoon (Blood Elf): 48,49 in Eversong Woods - West side of the dead scar near the city wall.
Undercity (Undead): Just inside the Ruins of Lordaeron in the courtyard. Definitely the easiest to find.
Orgrimmar (Orc, Troll): 47,17 in Durotar - East side of the road as you head south out of the gates of Orgrimmar.
Thunder Bluff (Tauren): 56, 18 in "Thunder Bluff" zone or 41,19 in "Mulgore" zone - just north of the North Great Lift in Thunder Bluff.

Alliance

Darnassus (Night Elf): 77,25 in Darnassus - Head East on the road and just past the Warrior's Terrace it is on your left.
Stormwind (Humans): 39,59 in Elwynn Forest - Just follow the road out of Stormwind South and it is on the West side before Goldshire.
Exodar (Draenei): 47,56 in Azuremyst Isle - Take the road East out of the Exodar and it is on the right as you reach Azure Watch.
Ironforge (Dwarf, Gnome): 54,39 in Dun Morough- Upon exiting the Gates of Ironforge turn left and follow the road around the side of the mountain.

If there are any mistakes please let me know.

The Day of the Dead is on the 1st of November, as noticed on the ingame calender.