Thunderfall is an encounter power available to Barbarians at 13th level[PH2:56]
Thunderfall
Barbarian Attack 13
You lower your shoulder, dipping your weapon beneath your opponent's guard and driving it up into the foe's body to knock it off balance.
Melee weapon
Special: When charging, you can use this power in place of a melee basic attack. If you charge and hit, the number of squares you push with this power equals your Constitution modifier
Target: one creature
Attack: Strength vs AC
Hit: 3[W] + Strength modifier damage and you push the target 1 square and knock it prone.