Absolutely fantastic card for Carolyn Fern. Carolyn can good value out of even subpar healing cards and loves Hallowed Mirror (0). Hallowed Mirror (3), however, seems custom-designed to annihilate Carolyn's weakness. Rational Thought is a nasty weakness that makes Carolyn useless at her primary purpose. Now, with Hallowed Mirror (3), if we have her signature Hypnotic Therapy in play, we can clear her weakness in 1 single turn by healing a mind-blowing four horror in a single turn. That is just spectacular. It is also fantastic if you are playing with her book strength/weakness. Foolishness is a great asset, but starts with 3 horror that must be healed before he's much use to you. That's one Soothing Melody - one action to get Carolyn her personal Crystalline Elder Sign. It's also great protection for her Black Wind weakness as well, as it makes you a much more reliable horror-healer.
Finally, let's consider that Soothing Melody's effect can be split between investigators/allies. Carolyn can (with hypnotic therapy), split the effect between herself and allies, to heal the team of 4 horror and earn the team 4 resources, all in 1 single round! Better yet, if there is currently only 3 horror out there to be healed, Carolyn can heal 3, earn 3 resources, and then reshuffle the card for later use.
And for one more bonus: this new Hallowed Mirror (3) does not cause Soothing Melodies to leave your deck when it does, so if something destroys the mirror, rest easy knowing your melodies are no longer vulnerable.
The versatility of this new hallowed mirror for Carolyn cannot be overstated and I see it becoming a staple upgrade for Carolyn decks.