You need two bars of thorium and a powerfully hot heat source to forge the key stem. You're on your own for the thorium, but I can help with the heat.
To our west in Un'Goro Crater is Fire Plume Ridge - a simmering volcano. The lava lake up top is hot enough to fuse thorium and bone together. Your gold has bought you a mold that simply requires you to dip it in the lake up top; the lava will take care of the rest.
That's it on my end. To finish your key, you'll need to talk to Dithers again.
Do you have the stem of the skeletal key finished? I'm truly sorry you had to bear the brunt of the fee to purchase the mold, but we must all do our part to counter the Scourge. Once you hand me the unfinished key, you'll be ready for the final item needed to complete its construction.
It won't be easy at all, but the work you've done to date in preparing for an assault on Andorhal will now start to pay for itself. You'll see.
We've got quite the costly skeleton key there, don't we <name>? Still, it remains unfinished; the time has now come to put some focus behind our creation. To that end, you're going to have to face the ultimate foe we've been working towards facing in Andorhal... the being that runs the show within the ruined city walls.
You must be on the shore of the lava lake that is in front of Blazerunner's cave. It won't work if you're not in the right spot, and it won't work if you're in combat.