The Sinister Calling achievement in WoW is an achievement based on seasonal events. This means that you can only get it during the Halloween events around Halloween.

The achievement is:

Get a Sinister Squashling pet

 

Get a holy helm

The Sinister Squashling pet is the small non-combat squash that follows you when you summon it. She has a pumpkin-shaped face and walks on vines from it. I collect gaming pets, so this is really what I was looking for in all my characters when I unlocked this achievement on a level 19 “tween” rogue that I rarely play but like to have around for novelty.

The Hallowed Helm is the big jack-o-lantern head that you’ve probably seen people use, but this isn’t the timed one that people can shoot at you (which is involved in another achievement), rather this is the rare one (blue ) item that is bound to your character’s soul and is a permanent helm that you can keep forever with no time limit.

Both of these items are rare items that are obtained through trick-or-treating with innkeepers in all cities. Each character can trick or treat once per hour. You have a chance to get a bag of goodies that contains items inside or to be “cheated”, which often puts you in a costume or turns you into a creature for a short period of time (also silences you) like a bat or a mini -Devil.

Goody bags have a chance of giving you candy, skins from the in-game races (also used for another achievement), costume wands you can use on party members (also an achievement), or these rare items mentioned above.

To get the achievement, you need to get both rare items on the same character. This is going to 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 consistently do this from the start of the Halloween 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, you just have to “farm” it on that particular character. But remember that you can only do this once per hour. You get a buff on your character when you trick-or-treat that will tell you how many minutes are left before you can do it again.