Ari'Tvovoz' Double Edged Shield

Extending your hand to your ally in peril, you draw the life force from yourself, creating a shield of necrotic energy around a creature you can see within range, raising their armor class to just exceed the attack made against them until the start of their next turn. Take 1d6 necrotic damage that cannot be reduced in any way for every number the creature being defended's armor class raises by, that cannot exceed 4d6 (or an increase of +4 to the armor class of the creature).

At Higher Levels. When you cast this spell at 3rd level or higher, the increase in armor class for the targeted creature can be raised by up to 2 (increasing the damage taken by up to 2d6) for every level above 2nd.