Entropy

You attempt to interrupt the flow of magical ether through a creature in the process of casting a spell. If the creature is casting a spell of 6th level or lower, its spell fails and has no effect. If it is casting a spell of 7th level or higher, make an ability check using your spellcasting ability. The DC equals 8 + the spell's level. On a success, the creature's spell fails and has no effect. However, the process of casting this spell has an effect on the target. 

The target must make a constitution saving throw with the difficulty equal to the caster's DC. 

Upon a failure to pass the DC, immolating spell energy explodes around them that once flowed through their own veins as it returns into heat energy in the atmosphere. The target takes 1d6 force damage and 1d6 radiant damage times the spell level. For example, a sixth level spell countered by Entropy would do 6d6 force damage and 6d6 radiant damage. Nostalgic

At Higher Levels. When you cast this spell using a spell slot of 7th 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.