You create a wall of necrotic energy on a surface within range. You can make a wall up to 60 feet long, 20 feet high, and 1 foot thick, or can make a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick. The wall is opaque and lasts for the duration. When the wall appears, each creature in its area must make a Constitution saving throw. A creature takes 4d8 necrotic damage on a failed save, or half as much damage on a successful one.   One side of the wall, selected by you when you cast this spell, deals 4d8 necrotic damage to each creature who ends their turn within 10 feet of that side or inside the wall. A creature takes the same damage when they enter the wall for the first time on a turn or end their turn there. The other side of the wall deals no damage.   Whenever a creature takes damage from the wall, you can use your reaction to gain temporary hit points equal to the amount of damage dealt.