Temporary Prism Wall

A crystalline barrier appears in response to the triggering attack. The barrier can either take the form of a 20 foot tall 60 foot long wall starting at a point you can see within range or a sphere centered on you with a radius of 30 feet. The wall protects creatures on the opposite side from the triggering attack and the sphere protects creatures inside of it.

If the triggering spell is of 5th level or lower or the ability is from a creature of CR 11 or lower, the barrier prevents all effects including damage. Otherwise the protected creatures gain the benefits of half-cover, advantage on any saving throws required not just Dexterity, and gain resistance to any damage sustained. 

The barrier vanishes immediately after use.