Bastion's armor of bones

You target a point within range. Causing a surge of undeath to emerge from its point. Creatures within 20 feet of the point must make a Constitution saving throw or take 4d4 piercing damage form bones being ripped from their bodies. Choose another willing creature within range. That creature gains a plus 1 bonus to AC for each creature damaged. This bonus lasts for the spells duration. if a creature were to be subjected to the AC bonus from this spell again. it loses the bonus from the previous casting