Mana Flare

3rd-level Abjuration
Casting Time: 1 action
Range: Self
Components: VS
Duration: (C) 1 minute

You destabilize the mana in the area around you. Whenever someone other than you casts a spell within 30 feet of you, they must make a Wisdom save. On a failure, a raw, magical backlash deals 1d6 arcane damage, plus 1d6 additional arcane damage for per level of the spell he cast, or half that damage on a success.
At Higher Levels. When you cast this spell at 4th level or higher, you begin to harness the energy of the field to a degree. At 4th level, allies have advantage on the saving throw. At 5th level, allies automatically succeed. At 6th level, mana flare no longer affects allies.

Source: Homebrew