Thundermaul

You gather an oncoming storm that coalesces at the head of your staff, This maul lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 3d8 thunder damage on a hit. In addition, when you hit a creature with this magic weapon must make a constitution save or be pushed 10ft away from you. 

If you roll a Natural 20 on the attack roll, you may cast the Chain lightning spell on the target as a free action.

At higher levels: When you cast this spell using a spell slot of 7th level or higher, the damage increases by 1d8 for each slot level above 6th.