Blackfire

You hurl a ray of Cold Black flame at a target you can see. Make a ranged spell attack, on a hit the target is set ablaze in a cold midnight colored flame that burns away their life energy.
At the start of each of the targets turns it must make a Constitution Saving Throw. On a failure the targets constitution modifier is temporarily reduced by 1d4, if the targets constitution score is reduced to 0 it dies, if another creature would enter within 5 feet of the target it must also make a Dexterity Saving Throw or be affected by the Blackfire. Those killed by Blackfire are irretrievably consumed; Not even a Wish can restore them. Blackfire cannot be smothered by conventional means, succeeding on 3 of the contitution saving throws extinguishes the flames. Blackfire will not burn in Antimagic or on those protected by Negative Energy, but it can be blown out by a Fireball, Lightning Bolt, or any similar 3rd level or higher Spell (the target still takes damage as normal). Those who survive Blackfire recover constitution at the rate of 1 point per hour.