since a bard's power is passive and i imagine more subtle than a Prince but equally as dangerous (long term danger rather than immediate) i imagine that a Bard of Heart would cause the slow corruption of souls around them. If you had "soul vision" you could see a black "corruption" spreading through the souls of living things in the Bard's vicinity. It basically does slow emotional damage until they completely break down and also damages their souls until they splinter

I mean both of them can slowly destroy over time, same as how Gamzee, a Bard of Rage, can just. Go Berserk and start murdering people violently.