The Mournful King is part of an encounter that Broadsword put in the game temporarily during the Halloween season in 2016, 2017, and 2018. The event is described here:
This trophy is exactly like the other skeleton trophies except bigger.
Many thanks to Buster for letting me take the screenshot in his house.
In a Nutshell
This page was first published on October 10, 2019 and last updated on October 21, 2019.