true blight weapon?

[Last]

how do you obtain or craft it?

[Despondency]

I think that's from upgrading your Hungmoon weapon.

[Zaphod]

Despondency has it right.  You get it in the normal chain of upgrading your weapon.  Is your weapon at MAX?  If so, you can mouse over the item you need to evolve your weapon, and it will tell you where you get it.