The Sinister Calling achievement in WoW is a seasonal event based achievement. This means you can only get it during the Hallow's End events around Halloween.
The achievement is to:
· Obtain a Sinister Squashling pet
· Obtain a Hallowed Helm
The Sinister Squashling pet is the little pumpkin non combat pet that follows you around when you summon it. It has a jack-o-lantern designed face and walks on its vines. I collect game pets so this is really what I was going for on all of my characters when I unlocked this achievement on a level 19 “twink” rogue that I rarely play anymore but like having around for novelty.
The Hallowed Helm is the big jack-o-lantern head you've probably seen people wearing but this is not the timed one that people can shoot onto you (which is involved in another achievement) but instead, this is the rare (blue) item that is soulbound to your character and is a permanent helm that you can keep forever with no time limit on it.
Both of these items are rare items achieved through “trick or treating” with the Innkeepers of all towns. Each character can trick or treat once per hour. You have a chance of getting a treat bag which contains items inside or of getting “tricked” which often puts a costume on you or turns you into a creature for a short period of time (also silencing you) such as a bat or a mini-Diablo.
The treat bags have a chance of giving you candy, masks of the game races (also used for another achievement), costume wands that you can use on party members (also an achievement) or these rare items listed above.
To get the achievement, you must obtain both of the rare items on the same character. This is going to require a bit more work. To do it, you must log each character in every hour on the hour (whenever possible) and trick or treat the innkeeper for a treat bag. If you do this consistently from the beginning of the 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 only have to “farm” it on that particular character. But remember you can only do it 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.