For this quest you kill loads of skeletons in the plaguelands they may drop Skeletal fragments.
Onvce you have 15 take them back to him he makes a magic pile of bones.

Goto Gadgetzen and talk with the goblin by the thorium brother-hood nightelf who is blacksmithing.

He charges you 15g and gives you a mold to pour some molten metal into to form the key.

Take the mold and 2 x Thorium bars to UnGoro crater's mountain fireplume ridge and melt them by the lake (lave lake) at the top.

Back in the days long before the key chain was introduced I threw away this key to save that precious space in my bags. Being a rogue I never needed the key anyway.

Now achievements have been introduced and this key is needed for https://vanillawowdb.com/?achievement=1187 The Keymaster.

The patchnotes (and a few blue posts in Blizzard's forum) say that keys can now be replaced by a blacksmith. Unfortunately they're not telling me which blacksmith they're talking about. For now I'm not even sure if the 3.0.2 patch notes are telling me you can get a lost key back now.

Would somebody point my mount in the right direction and give it a slap, please?