Mage "armor"

When a creature you can see hits you with a ranged attack while a spellcasting creature your size or larger is within 5 feet of you, you can use your reaction to try and shield yourself with the spellcaster.

The spellcaster must make a Strength saving throw against a DC equal to 8 + your proficiency bonus + your Strength modifier. On a failed save, the spellcaster takes the full damage of the attack instead of you. On a successful save, you take the damage as normal, and the spellcaster takes 1 point of damage of that type.

You can use this reaction twice per long rest, you regain all expended uses when you finish a long rest. You also regain one expended use when you finish a short rest.