A lot of people looking for it.
Haven't found it yet sadly, Best guess is either Treasure Caskets in Grounds of Valor Areas.
Could also be from VWNM (Void Watcher), but god i hope not.
Theres a lot of possibilities, Sadly our best bet is to go Farm Brown Caskets and Discover as many rare item from those as possible and rule them out.
Then theres only 2 options, a Quest (Unlikely, I didn't see any .dat Conversations regarding it), or VWNM.