Something I like in videogames and some fantasy tropes is the “Anti Bard”. Be it a Jester, a Dirge Singer or Doomsayer. They use the magic of music not to help but to bring despair and decay.
How do you do that mechanically? Simply saying someone does less damage or takes more damage doesn’t seem that interesting. Inducing specific fictional results might brake the fiction at some moments.
How would you do that?
Another idea is to use the normal Arcane Arts result an flavor that the advantage you create is because your enemy is weakened somehow.
(Quick point about why “does less damage” makes no sense. If I put -1d6 damage on an enemy and they never deal damage to something this was wasted. To be a fan of the character I am as a GM am incentivised to have the target deal damage to someone so that we can see that they deal less damage now – creating a zero sum result. It is not very interesting too)
(Quick point about why “takes extra damage” is bad: That is the same as giving extra damage to an ally)