Decastave

You hold a sliver of wood or bamboo while casting this spell and it turns into a pole of shimmering magical force. This pole can be used as a quarterstaff or sword and acts as a magical weapon. On a hit, the staff or sword does 2d6 force damage and 1d6 radiant damage. The opponent’s hit point maximum is reduced by an amount equal to the amount of radiant damage taken, and you regain hit points equal to that amount. The reduction lasts until your opponent finishes a short or long rest. The opponent dies if this effect reduces its hit point maximum to 0.

Source: The (Not Really) Complete Tome of Spells, page 147.