Mace of Dispater (UA)

Source: Unearthed Arcana 29 - Warlock & Wizard

Prerequisite: The Fiend patron, Pact of the Blade feature

When you create your pact weapon as a mace, it manifests as an iron mace forged in Dis, the second of the Nine Hells. When you hit a creature with it, you can expend a spell slot to deal an additional 2d8 force damage to the target per spell level, and you can knock the target prone if it is Huge or smaller.