Source: Unearthed Arcana 15 - Feats

The flail is a tricky weapon to use, but you have spent countless hours mastering it. You gain the following benefits:

• You gain a +1 bonus to attack rolls you make with a flail.

• As a bonus action on your turn, you can prepare yourself to extend your flail to sweep over targets’ shields. Until the end of this turn, your attack rolls with a flail gain a +2 bonus against any target using a shield.

• When you hit with an opportunity attack using a flail, the target must succeed on a Strength saving throw (DC 8 + your proficiency bonus + your Strength modifier) or be knocked prone.