Made this weapon based from the description from a book series I like. Hope it is balanced for an artifact weapon.

En-Vamir, the Hammer that Broke the Spine of the World

Weapon (maul), artifact (requires attunement)

En-Vamir was the weapon wielded by Koranir, the God of Strength and Stone, as a mortal man. The weapons title comes from his use of the maul to collapse an entire mountain range, preventing the escape of the Buried from their prison deep underground.

You gain a 1 to attack and damage rolls & deals an extra 2d6 thunder damage made with this magic weapon.

This weapon has 20 charges for the following properties. The weapon regains 1d8 4 expended charges daily at dawn.

Body of Stone: You channel the essence of stone through your body increasing your strength and making your flesh as hard as stone but you also gain its weight.

On your turn you can enter body of stone as a bonus action. You gain the following benefits while holding En-Vamir.

• You have resistance to bludgeoning, piercing, and slashing damage.
• You have advantage on Strength checks and Strength saving throws.
• Your Strength & Constitution score increase by 4, to a maximum of 24.
• Your disadvantage on Dexterity checks and Dexterity saving throws.
• Your speed is reduced by 10 ft.

Body of Stone last for 1 minute. It ends early if you are knocked unconscious or if you are no longer holding En-Vamir. When Body of Stone ends you gain a level of exhaustion. 

Breaking the World: You can slam En-Vamir on the ground to use its destructive power. You can use your action to spend 5 charges to create a small crater on the ground in a 15-foot radius centered on you. Each creature on that ground other than you must succeed on a DC 17 Dexterity saving throw. On a failed save, a creature takes 3d12 thunder damage and is knocked prone. On a successful save, the creature takes half as much damage and are not knocked prone. The area becomes difficult terrain until cleared, with each 5-foot-diameter portion requiring at least 1 minute to clear by hand. 

Additionally you can spend 10 charges to cast Earthquake (DC 17) without concentration or components. When you use this weapon in this way you cannot use the breaking the world feature for 200 days.

Power Strike: When you hit with a melee attack, you can expend up to 3 charges. For each charge you expend, the target takes an extra 1d6 thunder damage.