Improved Sack Smack

Description
You are getting good at swinging your sack in peoples faces. Having carried it around for a while you have developed an excellent sense of how to best make use of its weight to a point where it will even hurt your foes instead of just inconveniencing them.

Effects
Deals (X) damage per occupied inventory slot (improved by Strength)

Causes 25 knockback + a maximum of [Y] additional knockback depending on inventory load.