Well, PErsonally, i NEVER knew of this, Dunno if repost, but it isn't an exploit.
Today when I Was mourning that tommorow that no chance that I will get the squashling, when I was recieving the Basket Achievement..I took a Trick Or Treat..
Bam! I got a Sinister Squashling and completed the Sinister Call! And now I can finish the Hallowed By Thy Name.
I think it can drop too more rares, like the Hallowed Helm etc.
SS: