Basalt Cleaver (Patreon)

Weapon (battleaxe or greataxe), very rare (requires attunement)

A weapon forged of the hardest obsidian, the blade drips with molten magma when wielded. You gain a +2 bonus to attack and damage rolls you make with this magic weapon which deals fire damage instead of the normal slashing damage. While attuned to this weapon you are immune to damage from lava and can move across it as though you were affected by the Water Walk spell.

Mantle Breaker. As an action you can split the earth before you to create a river of lava 60 feet long and 15 feet wide. When you do so, each creature within its area must make a DC 14 Dexterity saving throw. On a failed save, a creature takes 6d8 fire damage, or half as much on a successful save. The area is considered difficult terrain, and when a creature makes contact with the area for the first time on a turn or ends its turn there, that creature takes 3d8 fire damage. When a creature moves across the surface, it takes 1d8 fire damage for every 5 feet it travels. After 10 minutes, the lava cools and solidifies. Once you use this ability it can't be used again until you finish a long rest.

Lava Spray. If you are within 5 feet of a source of lava, as an action you can use this weapon to splash a 20-foot cone of molten rock. Each creature in the cone must make a DC 14 Dexterity saving throw. On a failed save, a creature takes 4d8 fire damage, or half as much on a successful save.

Forged in the molten heart of a volcano, this jet-black weapon carries the heat of its origin inside itself. Though initially cool to the touch, at a moment's notice it roars alive with the power to transform the field of battle. Leaving smoldering slag heaps in its wake, the weapon scars the land with every swing.