Ken was upset at his roommates who were always either stealing his food, taking his stuff, or just... not paying their rent on time. So when he was walking his way back to the apartment he noticed a small stall on the side that read "April's Fool" and thought to himself "Yeah, I could pull a small prank on them. It's tomorrow."
April on the other side asked "Looking for anything in particular? Maybe some payback of sorts, general mischief, something silly for a significant other?"
Ken said "I dunno, maybe something for my roommates who just-" He didn't vocalize it, told himself to calm down, but the flash of rage he couldn't hide from April.
April said "How about a Fuck you too then? A treat for someone constantly being fucked over by others in a manner of speaking." She held out what looked like a pendant "Simply give it a trigger like someone taking something or doing some annoying act, and then when they Fuck you over... Well you can fuck them over as well."
Ken eyed the pendant suspiciously "As vague as that sounds, you're telling me that's magic, how much are you expecting me to pay?"
April took out a contract with no small print "Sign this contract and it's yours. You will not cause any lasting harm with this, and any emotional damage you cause you must personally deal with. Pranks are all good and fun, but it's good to temper that fun with the knowledge you need to clean up afterwards." The contract didn't seem magic exactly. Just some printer paper and ink. Just sign his name and get... it was a nice looking pendant. What's the harm? Ken signed his name and got the "Fuck you too".
When he got back in he had just enough time to set up his trap, assuming it even worked. He decided to make it trigger