Arcane Interrupt

When you witness another being casting a spell, you may attempt to cause that spell to fail.  A challenge between the casters ensues where you roll d20+spellcasting mod vs. their d20+spellcasting mod. 

If the slot at which you cast Arcane Interrupt is greater than the spell slot that the opponent was casting, you gain advantage on your roll.

If the slot at which the opponent cast their spell is greater than the slot at which you cast Arcane Interrupt, then the opponent gains advantage on their roll.

If your roll is EQUAL to or GREATER than the opponent, the opponent's spell fails. 

If your roll is GREATER than their roll, you regain the spell slot that you used to cast Arcane Interrupt.