Wind Strike

3rd level spell,

You can become one with the wind, almost incorporeal; flying through a creature attacking; dealing an extra 1d8 force damage on top of your melee attack. On a failed Dexterity save you knock the creature prone and a successful Dexterity save the creature is not knocked prone. Your movement doesn’t provoke opportunity attacks. You gain 20 feet of movement after the strike, whether you hit or miss. 

Size difference: If the intended target is a size above your own (small attacking large) and you want to knock it prone; to knock it prone you lose the rest of your extra movement, the extra 1d8 and you end up in front of the successfully hit now prone creature.

At higher levels - 5th, 8th, and 10th your attack increases by 1d8. At 8th level you add an additional creature within the extra 20 feet of movement and an additional creature at 10th. The additional creatures also have to make Dexterity saving throws.