Barnaby's Magic Mirror

You attempt to interrupt a creature in the process of casting a spell, and siphon away the spell's energy for your own immediate use. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it's casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + twice the spell's level. On a success, the creature's spell fails and has no affect.

When you successfully negate a spell this way, you may immediately cast it yourself without using an action, ignoring the spell's components. If the targeted creature was casting the spell at a a higher level, you cast the spell at whatever level the creature was attempting. You may choose new targets, etc. for the new spell. When you attempt to cast a spell this way, you cannot take actions until the end of your next turn.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the interrupted spell is successfully negated if it is more than one level lower than the level of spell slot you used.