You attempt to interrupt a creature in the process of casting the triggering spell.

The caster must roll a Constitution saving throw against your spell save DC to maintain concentration on the spell it is casting. On a failed saving throw, the creature fails to cast the spell (but retains the spell slot).

If the triggering spell is 3rd level or lower, the target fails the Constitution saving throw automatically.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the saving throw fails automatically if the triggering spell is equal to or less than the level of the spell slot you used.