Lowering the hit by one might not be enough to bring it in line with other themes. You could spend a minor action to gain +1 to your next attack. I think AA's suggestion of making it a buff or debuff, or inflicting a small amount of damage on a miss is more interesting.
Making the damage bonus +1 per tier would bring it in line with those kinds of feats. It's still an untyped bonus, which makes it mighty precious to strikers (radiant is a damage type, and I'm talking about bonus types here: feat bonuses, enhancement bonuses, item bonuses, class bonuses, racial bonuses, and so on).
As for the power, I'd make it a daily, and level 6 - if not higher - if you want to keep it as it is.
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