Force Spell (4e Feat)
From D&D Wiki
Force Spell [Metamagic]
One spell you know can deal force damage.
Prerequisite: 11th level, wizard
Benefit: Choose one wizard attack power that you know. Once per encounter you can choose to have that power deal force damage in addition to its normal damage type. The power gains the Force keyword for the purpose of this attack. Insubstantial creatures do not halve force damage inflicted by this power.