Backlash

When You or an ally are targeted by a spell, You may use your reaction to attempt to interrupt a creature in your sight from casting that spell.

Both you and creature must make contested rolls. player must use their dexterity modifier while creature must use their spellcasting ability modifiers for this contested roll.

if the contested roll is in the players favor, the spell explodes and creature takes the full damage or effect of the spell and is knocked prone.

if the contested roll is in the creatures favor, Backlash fails and the spell goes through. creature is immune to backlash for 24 hours.