When you or a creature you can see within 120ft is the target of an attack roll you may use your reaction to form a portal in local space-time, redirecting the strike to an enemy. 

After knowing whether the attack is a hit or miss, you may oppose an attack roll with your own ranged spell attack roll. On a failure,  the spell has no effect and the attack is resolved as normal. If you succeed, the attack is resolved against a different creature you choose within 120ft, using your successful spell attack roll.