Motu Monday: Power Sword for D&D 5th Edition REVISED

The Power Sword 
Magic weapon (longsword), legendary (requires attunement)

You gain a +4 bonus to attack and damage rolls with this magic weapon.

In addition, while attuned to the longsword you have advantage on saving throws against spells and other magical effects.  Further, when you roll a natural 20 on a saving throw against a spell, the creature who cast the spell becomes a target of the spell instead of you.