There's a new spell in the PHBII called Prismatic Mist (Sor/Wiz3) that might be interesting to sit a base effect on and then add a more CR appropriate kicker onto. Plus it ties in with new books, and who doesn't like excuses to buy new books? Well, besides people who can't afford them...The attached effect would probably have to be evil as heck to make up for the low level of that one.
Or, alternately I like the idea that it blinds, dazes/shaken/whatever, and does Con damage too. Maybe a low DC 15 save for permanent blindness, makes you helpless no matter what, and causes Con drain to allude to the death thing. Or level drain, level drain would be outstanding and different too. Something to make sure that the wussiness of being a fey doesn't nerf the "big scary bear" thing. If it can attack players and make them helpless THEN engage in melee...
Or, it breathes a cone of anti-magic that sucks up spell-levels. I don't know how you'd rationalize that, but it sounds like something fey would do.
After that, and including earlier suggestions of mine, my stream o consciousness brainstorm stops.