Anders' Radiant Hammer

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails.  On a hit, the target suffers the attack’s normal effects, and the caster becomes shrouded in radiant energy.  Until the end of your next turn, the first time a hostile creature performs a melee attack against the caster or a friendly target within 5 feet of the caster, they take radiant damage equal to your spellcasting ability modifier. The spell then immediately ends. This spell's damage increases when you reach higher levels

At 5th level, the melee attack deals an extra 1d8 radiant damage to the target, and the radiant damage to the second creature increases to 1d8 + your spellcasting ability modifier. Both damage rolls increase by 1d8 at 11th level and 17th level.