Bats are common familiars for warlocks and those who prefer subtle magic and stealth.

Senses blindsight 5Speed 2, fly 6 (hover)Constant Benefits* You gain a +2 bonus to Perception checks and Stealth checks.Active BenefitsSee the Unseen: As a minor action, you cause enemies adjacent to your bat familiar to lose the benefits of concealment (but not total concealment) against you until the end of your current turn.

Published in Arcane Power, page(s) 140, Heroes of the Feywild, page(s) 131.