Virulence

You unleash a plague in a 30-foot radius sphere from a point within range. Each creature of your choice in that area must make a Constitution saving throw. On a failed save, it takes 10d8 necrotic damage, or half as much on a successful save. If the target fails the saving throw, its hit point maximum is reduced for 1 hour by an amount equal to the necrotic damage it took. Any effect that removes a disease allows a creature's hit point maximum to return to normal before that time passes.

A creature killed by this damage rises up as a Ghast at the start of your next turn.

On each of your turns, you can use a bonus action to mentally command any Ghast within 60 feet of you that has been made as a result of this spell. If you control multiple creatures, you can command any or all of them at the same time, issuing the same command to each one. You decide what action the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until the task is complete or is told to stop.

The creatures are under you control for 8 hours, after which they stop obeying any command you have given them and become hostile.