Elemental Evil: Tremblor, a magic pike for D&D 5th Edition

In my Princes of the Apocalypse, I've taken to creating some magic items to seed my campaign with:

Legend holds that Barrage took this weapon from a Templar of Grumbar that he disembowled in the Great Gray Lands of Thar.
He often plunges the weapon deep into the earth after felling a foe and mocking the Lord of Earth

TREMBLOR
Magic weapon (pike), rare (requires attunement)

You gain a +1 to attack and damage rolls with this magic weapon.

In addition, when you score a critical each enemy within a 10 foot radius takes 1d6 bludgeoning damage and must make a DC 12 Dexterity saving throw or be knocked prone.

Finally, as a bonus action the ground in a 20-foot radius centered on you becomes difficult terrain. Once you use this ability, you must finish a short or long rest before you can use it again.