Black Mist

 Misty Form. The mist can occupy another creature’s space and vice versa. In addition, if air can pass through a space, the mist can pass through it without squeezing. Each foot of movement in water costs it 2 extra feet, rather than 1 extra foot. The mist can’t manipulate objects in any way that requires fingers or manual dexterity.

Take Over (Recharge 6 on d6)  - If the black mist is within 5 ft of a  medium size creature or smaller with at least 1 hit point it can try to take over the creature. The target must succeed on a con save of 17, on a success nothing happens, on a failure the black mist goes inside the creature and controls the creatures turns for one minute or until the targeted creature falls to 0 hit points. While the black mist is inside the creature, the black mist takes no damage from attacks and spells, but can be the target of spells and abilities. While inside the creature the black mist can use the creatures attacks, but can't use the creatures class abilities or cast the creatures spells, but can still cast their own spells.

Can cast each spell once per long rest, DC 17 -  Sleep, Silence, Blindness/Deafness, Darkness, Hold Person, Hypnotic Pattern, Enemies Abound, Dispel Magic, Confusion, and Dimension Door