Please forgive how sloppy this is written, i'm half asleep; but for instance similar items to this...
+2 Shield of Narcissus.
"It's smooth surface is polished to a mirrored perfection."
Encounter Power
Trigger: Attack made against AC.
Effect: As an immediate reaction, player raises shield to provide full cover against attack, granting an additional 3 AC until the beginning of player's next turn. Taking cover under the shield causes player to be dazed for .5x Cha mod turns rounded down.
There are a few things I would refine if I were to allow it in my game.
First, it appears as if you have an enhancement bonus, but I'll assume what you really mean is that this is a large shield (which grants +2 shield bonus to AC and to Reflex)
In order for this to do what it seems to be designed to do, it would have to be an immediate interrupt, rather than a reaction.
Because it is a shield, I would change the trigger to "When you are hit by an attack that targets your AC or Reflex."
Because superior cover is not a shield bonus (and, therefore stacks with the shield bonus) and because the penalty for using this power is severe, I would go with the full -5 penalty to the attacker's roll in addition to the normal shield bonus. But, then, for flavor's sake, it would be good to see high-Cha characters using this shield, so I'd probably drop the superior cover feature and instead penalize the attack by the wielder's Cha mod.
Under no circumstances would I keep the penalty as is. Currently it penalizes high-Cha characters who try to use it but has no penalty at all for the Cha-dump stat tank who would actually be using this this shield (as currently written). Okay, I'm really not big on the idea of including a penalty at all (unless it's cursed), but, assuming I keep a penalty for the sake of flavor, it would be a lot flatter in application: "dazed until end of next turn," or something.
I'd probably also change the power to a daily.
And, for flavor, I'd give it a property: "+2 item bonus to diplomacy and bluff checks while wielding this shield."