Tartarus-ium is a hybrid word formed by the following:
Tartarus - In Greek Mythology this is where the defeated Titans were imprisoned.
Titanium - The real life metal with all of its strong and corrosion resistant properties.
For this fantasy setting, let's assume it names a special variant of Titanium that is only found in our game.
Let us also assume that that because Eyrun who is a kin to Cloud Giants and created the Havenspass area with her backstory has a Greek origin sounding character name, that using Tartarus is fair since it is a nod to the Titans in Greek mythology.
In this campaign what we know about Tartarusium is that there's at least one source local to the Havenspass area in what will be known as the Tartarusium Ravine. It is here that Havenspass “local” Lapata sources it.
If you research how it is mined and processed, then you'll find this involves more of an open pit style scenario suited to a ravine, which works for the Havenspass region of the game. How Lapata processes it after that or what Lapata's ultimate plans are are of no concern. Only that this is how and where it first enters the campaign.