The Sinister Calling achievement in WoW is an achievement based on seasonal events. This means that you can only get it during Hallow’s End events around Halloween.
The achievement is:
Obtain a Sinister Squashling Pet
Obtain a Holy Helm
The Sinister Squashling Pet is the little non-combat pumpkin pet that follows you when you summon it. It has a jack-o-lantern-patterned face and walks on its vines. I collect in-game pets, so this is really what I was looking for with all of my characters when I unlocked this achievement on a level 19 rogue “youngster” who I hardly ever play anymore, but who I like to keep around for novelty.
The Hallowed Helm is the big Jack-o-lantern head that you’ve probably seen people use, but this is not the one that people can time-shoot at you (which is involved in another achievement), rather this is the rare (blue) element that is linked to the soul of your character and is a permanent helmet that you can keep forever without a time limit.
Both items are rare items that are obtained by “trick or treating” with the innkeepers of all the towns. Each character can ask for candy once an hour. You have a chance to get a goody bag that contains items inside or is “tricked” by you, often putting you in a disguise or turning you into a creature for a short period of time (also silencing you) like a bat or a mini-Devil.
Goodie bags have a chance to give you candy, in-game racing skins (also used for another achievement), costume wands that you can use with party members (also an achievement), or these rare items listed above.
To get the achievement, you need to get the two rare items of the same character. This will require a bit more work. To do so, you must check in each character every hour on the hour (whenever possible) and trick or treat the innkeeper for a bag of goodies. If you do this consistently from the beginning of Hallow’s End events to the end, you will greatly increase your chances of getting this achievement on at least one of your characters.
If you only want it on a specific character, then you just have to “farm” it on that particular character. But remember that you can only do it once an hour. You get a buff on your character when you do tricks or treats that will tell you how many minutes are left before you can do it again.