You are only offered this quest if you have a in your inventory when you talk to the NPC. It does not require a particular reputation level with Thorium Brotherhood.
The undersigned hereby declares that the Sulfuron ingot they are delivering to the Thorium Brotherhood (TB Ltd.) is genuine and not a facsimile thereof. The undersigned also acknowledges that any harm that should come to <him/her> as a result of attempting to build, forge, find, create, sell, or eat Sulfuras, is not the responsibility or fault of TB Ltd.
By signing this contract, you are agreeing to all terms listed herein.
Do we have a deal?
It has been a pleasure doing business with you, <name>! On behalf of TB Ltd., I wish you luck in your search for Sulfuras!