If you don’t want to kill Priscilla and want the dagger, here’s a guide.
1. You’ll need the Lordvessel, so go kick O&S’ butts
2. Rest at the painted world bonfire
3. Beat painted world and JUMP OFF, do not attempt to get the tail yet.
4. Go to any bonfire that can be warped from and warp to the painted world bonfire (do not rest on it, if you did, repeat step 3)
5. Go to Priscilla and cut off her tail, after that bone out.
6. When you’re out, absolve sins at Oswald.