So this begs the questions: Where did they go? And what is this weapon?
If it's this amulet, it's either the one Orcus was looking for, which I cannot find what it might be called, or it's something new entirely. There aren't many (see also, any) artifact amulets. If it's something else, then what could it be? That would depend on where they went. If they went to an outer plane, then it could be anything really: an infernal siege engine, a demon ship from the Abyss, or a Marut. They more than likely went somewhere hospitable, though an upper plane seems unlikely, especially with a lack of "weapons" of note that could be brag-worthy. The Latanese tend to be Gondsmen, and he resides on the Outlands, what if they went there and crafted something really freaking nifty?
Or, something more crazy: They ended up in another campaign world. What if they ended up in Eberron and brought back one of their niceties? If it's a "weapon," I'm inclined to believe it's something really special, maybe a warforged juggernaut? My first thought would be a creation forge, but not necessarily a weapon....but, what if it's something big, really big...maybe the theoretical super weapon that may have cause the Mourning?
My thought process really likes that idea.