A Suburban Summoning (Imp #0.8)

Halloween is all fun and games until you're summoned into a closet.

Forced to help two children rid their father of a murderous girlfriend to satisfy the terms of her summoning, Sam finds neither the girlfriend, or the father, are quite what they seem.
This short story was originally published in 2015 under the title Full Moon Mischief in the Nightshade anthology.