Cinderfall
Cinderfall was reforged from the hilt of a broken warhammer dragged out of a collapsed volcano by a giant-blooded reaver named Ossian the Unfinished. The weapon is said to carry a memory of the mountain's collapse, every time it strikes true, observers swear they hear the distant thunder of a peak giving way. Those who have survived Ossian's descendants describe the blade not as swung, but as dropped, again and again, like a sentence the earth never finished saying.
Mechanics
A massive greatsword wreathed in volcanic fire that hits harder while you rage and detonates in a concussive burst once per day.
Volcanic Weight
This greatsword deals an extra 1d6 fire damage on each hit. The blade is unnaturally heavy, it counts as a two-handed weapon even if an effect would otherwise allow you to wield it in one hand.
Rage Ignition
While you are raging, the extra fire damage from Volcanic Weight increases to 2d6, and the first time you hit a creature with this weapon on each of your turns, the target must succeed on a DC 14 Constitution saving throw or be stunned until the end of your next turn. On a successful save, the target is instead incapacitated until the end of its next turn. This condition ends immediately if the target takes damage from any source other than this weapon.
Earthshatter
Once per day, when you hit a creature with this weapon, you can choose to bring the blade down with earth-splitting force. The target and each creature within 10 feet of it must succeed on a DC 15 Dexterity saving throw. On a failure, a creature takes 4d8 fire damage and is knocked prone. On a success, a creature takes half as much damage and is not knocked prone.
DM Notes
Balance considerations: Rage Ignition's stun-on-hit is powerful, but it requires the barbarian to already be raging, is limited to once per turn, and allows a Constitution save (DC 14 is achievable by most mid-tier monsters). The fallback incapacitation on a successful save keeps the effect meaningful even against high-Constitution enemies without making it a guaranteed lockdown. Earthshatter's 4d8 fire damage in a 10-foot radius is roughly equivalent to a 3rd-level spell, appropriate for a once-per-day rare weapon. The fire damage type is a known resistance vector, consider placing this weapon in a campaign where fire resistance is uncommon, or populate encounters with creatures that have vulnerability to fire for high-drama moments. The stun condition from Rage Ignition and the prone condition from Earthshatter can synergize dangerously in the hands of a high-level barbarian, if the barbarian reaches levels where Extra Attacks allow multiple hits per turn, watch the Rage Ignition stun closely; it only fires on the first hit per turn, which caps the lockdown potential. The attunement restriction is mechanically warranted because Rage Ignition explicitly triggers off the barbarian's Rage class feature.
Generated with: Level 5 · Barbarian · Path of the Berserker · Weapon (Greatsword) · Big smash · Heroic · Fire · Rare