The Sinister Calling achievement in WoW is a seasonal event based achievement. This means that you can only get it during Halloween events around Halloween.
The achievement is:
Get a Squashling Sinister pet
Get a holy helm
The Sinister Squashling pet is the small non-combat pumpkin that follows you when you summon it. She has a pumpkin-shaped face and walks on her vines. I collect gaming pets, so this is really what I was looking for on all of my characters when I unlocked this achievement on a level 19 “youngster” rogue that I rarely play but like to keep around for novelty.
The Hallowed Helm is the big jack-o-lantern head you’ve probably seen people wear, 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 the innkeepers in all cities. Each character can trick or treat once per hour. You have a chance of either getting a goodie bag that contains items inside or being “tricked”, which often puts you in a costume or turns you into a creature for a short amount of time (also silences you) like a bat or mini -Devil.
Goodie bags have a chance of giving you candy, skins from the game’s races (also used for another achievement), costume wands that 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 need to check in each character every hour on the hour (whenever possible) and trick or treat the innkeeper to get 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 it once per hour. You get a buff on your character when trick or treating that will tell you how many minutes are left before you can trick or treat again.