From publisher blurb:
"That Boss" Your Group is Sure to Remember
A "That Boss" is a new type of boss fight that will challenge players in a way that few published products ever have. Drawing influence from phased boss fights, prioritizing actions that aren't just "I try to do damage", and forcing dynamic movement from characters are just some of the things that a "That Boss" offers.
The Burnished King was a ruler who fell prey to his obession with beautiful liquid metal discovered in a nearby mine. So great was his material lust that he eventually threw himself into a vat of the molten mercury. Great treasure await in the haunted ruins of his gilded domain.
The Guilded King can be inserted into nearly any fantasy setting as a little known secret from a bygone age, an incredibly popular rumor, or a well-known place that is oft avoided by the sensible. Build an entire adventure around this battle or find a way to refluff it an insert it into nearly any adventure!