Well, the weapon part would come from "it is what it is", that won't cost extra.
The next power sounds pretty much like a differently worded channelling. Call it "Prowess of the huntsman" and you should be done. Maybe have it use survival and weapons instead of conviction and discipline, to make it fit the hunter motif.
The last one I can see multiple ways to go.
- First a simple stunt. Get symbolic link, perform ritual, get +2 to damage.
- Next, it won't get you exactly a +2 weapon damage, but you could model something similar to the Righteousness power. The "Potent Prayer" part is easy enough, simply name your target and invoke your high concept to let survival complement any action that have to do with pursuing the target. I don't have an idea for the "Desperate Hour" part, though, but there should be plenty of options. you could for example put a supernatural sense in there instead, that is in line with tracking your prey.
- And finally, you could simply take the ritual part to the "Prowess of the huntsman" power above. At that point, you can put it together as sponsored magic, which would fit perfectly into the Erlking story of the item. You would not exactly get a weapon 4 here either, but you should be able to do a whole lot with it. If you keep the actual magic narrow enough, it should not be a problem to use survival and weapon as I suggested above.