Furious Flail (Patreon)

Weapon (flail), very rare (requires attunement by a barbarian)

This magic stone flail has the heavy property. If you're raging while holding the flail, its volcanic nature reacts to your fury. When it does, the basalt handle extends, giving the weapon both the reach and versatile (1d10) properties. In addition, while you're raging, the flail deals an extra 1d8 fire damage to any target it hits. You take 1d4 fire damage when the flail transforms in this way and again at the start of each of your turns if you're holding the flail. The flail returns to its normal form and cools if you are no longer holding it at the end of your turn or if your rage ends.

Volcanic Fury. You can use an action to smash the flail into the ground at a point within the weapon's reach. When you do, the ground erupts, spewing a mixture of earth and fire in a 20-foot cube centered on that point. Each creature other than you in that area must make a DC 16 Dexterity saving throw. On a failed save, a creature takes 2d12 bludgeoning damage and 2d12 fire damage, or half as much damage on a successful one. In addition, the ground in that area becomes difficult terrain until cleared. It takes 1 minute to clear an affected 5-foot-square space of the rubble by hand. Once this property of the flail has been used, it can't be used again until the next dawn.

Foolish people believe nature to be kind and patient. I know a different kind of nature.