Flail Expertise

Benefit: You gain a +1 feat bonus to weapon attack rolls that you make with a flail. This bonus increases to +2 at 11th level and +3 at 21st level.
    In addition, when you hit with a melee weapon attack using a flail and the attack lets you slide the target, you can knock the target prone instead of sliding it.

Published in Mordenkainen's Magnificent Emporium, page(s) 23.