In Princes of the Apocalypse, there is a black arrow found pinning a skull to a tree in The Last Laugh side trek. Here is how the skull has taken shape in my campaign:
Wondrous item, rare (requires attunement by a wizard or sorcerer)
Once per day, this figure of wondrous power may be activated to conjure a specter for up to 4 hours.
In addition, when you cast the chill touch cantrip, you gain a +2 to your damage roll.