Dance of Thunder

The sound of Thunder echoes with your steps, and crashes with each strike of your weapon.  Increase your movement speed by five feet. You must maintain movement while this spell is active, taking at least five feet of movement every round.  This five feet of movement does not trigger Attacks of Opportunity,. 

So long as you can maintain concentration on the spell, your melee weapon attacks deal additional thunder damage equal to 3d4 + your casting modifier. In addiition, you may make an extra melee weapon attack per round.

At the end of the spell, your movement speed is halved and your attacks and dex saves will be made at disadvantage for one round.