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Vicious Mockery
Bard Attack 1

You unleash a string of insults at your foe, weaving them with bardic magic to send the creatures into a blind rage.

Ranged 10

Target: One creature

Attack: Charisma vs. Will

Hit: 1d6 + Charisma modifier psychic damage, and the target takes a -2 penalty to attack rolls until the end of your next turn.
Level 21: 2d6 + Charisma modifier damage.

Vicious mockery is an at-will power available to bards at the 1st level.[PH2:69]

Battling with words, no better the worse.

This is an excellent choice, being recommended as the second best power for the bard. It can be combined with other feats, such as Beguiling Enchantment and Psychic Lock to further increase the penalty (with these two feats, a -4 penalty to attack in general, or -6 against the attacking bard.) It can also be combined with a mark effect to provide a further penalty.