I'd start by answering some questions:

1) Who is your patron, and why is it making a deal.
- It's the weapon of a dirty, and it wants to accomplosh the diety's goals. There's a secret society opposed to the diety that it wants you to kill, and it will let you know when it wants something.
- It's a legendary weapon that has been fragmented across the planes and it wants you to recover and reforge it.
- It's a reified idea of battle/protection/whatever that you've tapped into.
- it's the weapon a legendary hero/conquerer/whatever, and it wants you to finish her mission and protect/conquer/whatever Ankh Morpork

2) How did you contact it?
- Dreams/Stumbled into a fragment/Family bloodcurse from when your great grandfather survived a wound from it/took shelter in a forgotten shrine/etc.

3) Why did you take the deal?
- Up to you