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White wolves
Bard Attack 23

You lament the lives lost to the white wolves. So stirring is your song that it draws the howling pack from the spirit world.

Target: each enemy in the blast

Attack: Charisma vs. Fortitude

Hit: 2[W] + Charisma modifier cold damage, and the target falls prone.

Effect: The blast creates a zone that lasts until the end of your next turn. Any enemy that ends its turn in the zone takes 5 cold damage, and it is immobilized until the end of its next turn.

White Wolves is an encounter power available to bards at the 23rd level.[Dr396:31]

This is rated as an excellent choice. It is a combination of a weapon attack against fortitude, knocking targets prone, and forcing enemies to move (risking Opportunity attack or other effects) or become immobilized.