Astrobleme

This large, powerful hammer is forged from heavensteel, an ancient metal from the stars above believed to be gifted by the gods. You gain a +1 bonus to attack and damage rolls made with this magic weapon. The hammer is capable of dealing crushing blows to metal armors and shields. After dealing damage directly with the hammer, metal armor and shields takes a permanent and cumulative -1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed.

Starfall. You can call upon the might of the heavens from where the metal came from to empower the hammer. Using an action, you can slam the hammer into the ground like a meteor impact. Each creature in a 30-foot-radius originating from you must make a DC 15 Dexterity saving throw. On a failed save, a creature takes 4d6 bludgeoning damage, 5d6 fire damage, and 1d6 radiant damage. On a successful save, the creature takes half as much damage. The area becomes rough terrain. This feature cannot be used again for 7 days.

Reforge. Over the course of 8 hours, this hammer can be enhanced with an additional 10 pounds of raw heavensteel. The creature reforging the hammer must have proficiency with smith's tools and a Strength of 20 or higher to work with the metals. Heavensteel is impervious to normal flames and must be melted with magical fire. Once reforged, the hammer gains the following properties:
- You gain a +2 bonus to attack and damage rolls made with this magic weapon instead of +1.
- You can use the Starfall feature every 48 hours.
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No one wished for it, no one asked for it. Just one day it fell from sky. A gift from the gods? Or an accident? Who's to say. All we know is that it impacted with the force of godly mighty hammer.

Proficiency with a Maul allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage, Combat, Heavy, Two-Handed, Topple