Success will depend on your music and discord skill as well as the instrument type (Normal, exceptional, slayer).
According to the Stratics Barding Difficulty/Success Calculator, and assuming you have 120 music, you will have a two percent chance of discording a dark father with:
- a normal instrument at 116 discord skill
- an exceptional instrument at 111 skill
- and a slayer instrument at 106 skill.
If you are at max 120 music and discord, then you will have:
- a 10% chance with a normal instrument
- a 20% chance with an exceptional instrument
- and a 30% chance with a slayer instrument.
The calculator on Stratics can be found
here if you want to test it with different skills, monsters, etc.
Safe Journeys!