Stalagmight Geode

This snapped-off rock formation has a cache of magical, growing crystals inside it. You gain a +1 bonus to attack and damage rolls made with this magic weapon. When you hit a creature with this weapon and roll the maximum number possible on the weapon's damage die, the crystals rapidly grow and increase the weapon's damage die by one size, up to a d12. For example, when you roll an 8 on the weapon's normal d8 damage die, its damage die magically becomes a d10 after the attack. Immediately after you roll a 12 on a d12 damage die in this way, each creature within 5 feet of the greatclub (other than yourself) must make a DC 15 Dexterity saving throw. A creature takes 1d12 piercing damage on a failed save, or half as much damage on a successful one, as the weapon's crystals explode into shrapnel. Once the crystal explodes, the weapon's damage die becomes a d8 again.

Two-Handed: This weapon requires two hands when you attack with it.

Proficiency: simple, greatclub Source: The Griffon's Saddlebag: Book Two p. 170 (Third Party)