The enchantment works the best for him because of this trade-off. He calls it Pain to Power, playing the guitar in the pentatonic scale will unleash a physical blow to his targets. The longer he plays, the more pain he endures, but the more powerful the attack is.
His job is hunting demons. He uses his enchanted guitar to deal damage to magical creatures, and being a magical creature himself, using that guitar causes him a bad kind of pain.