You attempt to interrupt a creature in the process of casting a spell. Make an ability check using your spellcasting ability, adding your proficiency bonus. The DC equals the creature's spell save DC, if the spell the creature cast was higher then the level this spell was cast at, minus the difference between the two from your ability check, if the spell the creature cast was lower then the level this spell was cast at add the difference between the two to your ability check. On a success, the creature’s spell fails and has no effect.