Inspire slowing

This is a form of bardic music.

A bard can use song or poetics to inspire slowing in his enemies. All enemies that come within 20 feet of the bard have to make a Will save (DC 13 + = Bard level + CHA modifier) otherwise they will be slowed down. Initially they are slowed down 15% but at 16th level they are slowed by 30%. There are no further penalties besides movement, unlike the slow spell.