Apart from +100% damage and looks/art, rest of the things pretty much are not impressive, as per my first impression.
Getting 12 gems slower is almost a "Not to pick card" for many light decks, unless you have Mythic girl cards with 2K+ damage, I think. I am still kind of pondering whether this ability will help or hamper Light Decks.
However, If ability is changed (in hypothetical future update) to making Dark skills 12 gems slower instead - then this would directly be "God Tier" for Light decks and vs Dark Deck opponents.