Axe of the Raging Tempest

In order to craft this weapon, the head must first be forged out of cold iron, melted by alchemist's fire. Then the axe must be struck by lightning from a naturally occurring storm, imbuing it with the raw energies of fire, lightning, and ice. 

While your storm aura is active, the weapon damage type changes to fire, lightning, or cold, depending on the chosen environment, flaring up with the power of the storm. This weapon, and its effects detailed further below, bypass all resistances and immunities to fire, lightning, or cold.

Once per long rest, you can tap into the elemental power imbued within the axe to empower your attacks. For 1 minute, the radius of your aura is 30 feet, and empowered with an extra effect as described below. If an effect requires a saving throw, the DC equals 8 + your proficiency bonus + your Constitution modifier.

Desert  When a creature takes damage from your aura, it emanates blinding light. Each target hit must make a Constitution saving throw or become blinded until the end of their next turn.

Sea When a creature takes damage from your aura, it becomes wreathed in lightning until the start of your next turn. When the target is hit by an attack while it is wreathed in lightning, the energy explodes outward. The target, and creatures of your choice within 5 feet of it, takes 2d10 thunder damage on a failed save, or half as much damage on a successful one.

Tundra When a creature gains temporary hit points from your aura, the ground in a 10-foot radius around each creature is icy and is difficult terrain for creatures hostile to them. Additionally, their weapon attacks deal an extra 1d6 cold damage.

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

Notes: Storm Herald Barbarian, Damage, Buff, Debuff, Combat, Subclass Feature, Heavy, Two-Handed, Cleave