Bor's Weapon

Maul of Earth Tremor
+2 maul that on a 20 has the following effect, You cause a tremor in the ground in a 10-foot radius.
Each creature other than you in that area takes 1d6 bludgeoning damage and is knocked prone.
If the ground in that area is loose earth or stone, it becomes difficult terrain until cleared.

Any way that could be adjusted to each enemy creature in the area? Or at least give allies a saving throw chance?