Belt of Aggressive Grappling

While wearing this belt, if you are Grappling a creature up to 1 size larger than you, you can choose to attach by swinging the grappled creature as a weapon.  You are proficient with it.
The damage die scales with the "weapon size":
1d4+STR for small creatures, 1d6+STR for medium, 1d8+STR for large, etc.
The "weapon" suffers the same damage it deals.
You can use this feature once per long rest.
Proficiency with a shortsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.