Downcast

If you witness a creature casting a spell higher than it's base level, you can use your reaction to impose a saving throw on them, on a failure the targeted spell's level is reduced by one. this can not be used to make a spell be cast below its initial level. On a success against the save this spell has no effect. 

At Higher Levels Casting. For every spell level above second that this spell is cast at, you can reduce the level of the spell by a factor of one. For example, if you cast this at 3rd level after witnessing a 5th level fireball being cast, and they fail the save, the fireball is reduced to 3rd.