A vortex of black fire appears in a 20-foot-radius sphere centered on a point within range. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it. The cloud moves 15 feet directly away from you at the start of each of your turns. It ignites flammable objects and rots plants in the area that aren’t being worn or carried and spreads around corners.   When the cloud appears, each creature within it must make a Dexterity saving throw. A creature takes 4d8 fire + 4d8 necrotic damage on a failed save, or half as much damage on a successful one. Additionally, on a fail, their hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest.   A creature must also make this saving throw when it enters the cloud for the first time on a turn or ends its turn there.   The black fire spreads around corners. It ignites flammable objects and rots plants in the area that aren’t being worn or carried.