Valkaryn's Counterspell

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 4th level or lower, its spell fails and has no effect. If it is casting a spell of 5th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and the Creature takes a backfire equal to half the dice that would have been rolled if it had succeeded against you.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used. If you cast this spell at 6th level or higher, the backfire adds another dice for every level you cast it