Hammer of Thunderbolts: 4d6?

So I pulled a Hammer of Thunderbolts from a Ancient Green Dragon's hoard recently, but when you important into the sheet it's rolling 2d6 'secondary damage'. None of my group can figure out why, the rulebooks don't exclusively state it does bonus damage in any way - though it was a 4d6 Large Warhammer in previous editions. API bug?

Hello Jadon,  I believe you are inquiring about this Hammer of Thunderbolts . As such, in D&D 5E the Hammer of Thunderbolts is considered a maul. A maul's base weapon damage is 2d6. It also has a +1 to Attack and Damage Rolls for this weapon. However, beyond that, there are no other damage modifiers to this weapon.  We hope this information helps. Let us know if you have any further questions. Thanks!