Rattling

If a creature has training in Intimidate and deals damage with a power that has the rattling keyword, the target takes a -2 penalty to attack rolls until the end of the user’s next turn. A target that is immune to fear is not subject to this penalty.

Published in Martial Power, page(s) 7, 73, Player's Handbook 2, page(s) 220, Martial Power 2, page(s) 57, Rules Compendium, page(s) 119, 315.