Talisman of the Sign (Patreon)

"... ah yes now this one is for sale, but I wouldn't recommend wearing it unless your mind is ready to see ungodly horrors and the spaces in between... I know some people are into that kind of thing. If it helps, this one is on discount"

DM notes - you want something creepy to give your party... here it is. Eldritch sign on one side. Organic squishy parts on the other. What's not to love. Best of all, if you really want to force this story, just have the pendant appear in someone's bag...