Bartzabel

Legendary Resistance (3/day). If Bartzabel fails a saving throw, it can choose to succeed it instead.

Undying Resolve. If damage reduces Bartzabel to 0 hit points, it must make a Wisdom saving throw with a DC of 5 + the damage taken, unless the damage is from radiant or critical hit. On a success, Bartzabel drops to 1 hit point instead.

Dual State. Bartzabel exists upon the Material plane in one of two forms and can switch between them at will. In its material form, it has resistance to damage from nonmagical weapons. In its ethereal form, it is immune to damage from nonmagical weapons. The creature's ethereal form appears as a dark purple outline of its material form, with a blackish-purple haze within. Bartzabel, while in ethereal form can move through the air as though it were water, with a fly speed of 40 feet.

Form Swap. As a bonus action, Bartzabel can alter between its material and ethereal forms at will.

Void Aura. Bartzabel is surrounded by a chilling cloud. A living creature that starts its turn within 5 feet of it must make a successful DC 18 Constitution saving throw or be slowed until the start of its next turn. In addition, any creature that has been diseased Bartzabel or its zombie servants takes 14 (4d6) cold damage every time it starts its turn within the aura.

Infecting Telepathy. If a creature communicates telepathically with Bartzabel, or uses a psychic attack against it, Bartzabel can spread its disease to the creature. The creature must succeed on a DC 18 Wisdom saving throw or become infected with the same disease caused by Bartzabel's tentacle attack.

Multiattack. Bartzabel makes three tentacle attacks or three withering touches, depending on what form it is in.

Tentacle (Material Form Only). Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit:  12 (2d6 + 5) bludgeoning damage and 21 (4d6) acid damage. If the creature is hit, it must make a successful DC 18 Constitution saving throw or become diseased. The disease has no effect for 1 minute; during that time, it can be removed by lesser restoration or comparable magic. After 1 minute, the diseased creature's skin becomes translucent and slimy. The creature cannot regain hit points unless it is entirely underwater, and the disease can only be removed by heal or comparable magic. Unless the creature is fully submerged in water or frequently doused with water, it takes 6 (1d12) acid damage every 10 minutes. If a creature dies while diseased, it rises in 1d6 rounds as a nihilethic zombie. This zombie is permanently dominated by Bartzabel.

Withering Touch (Ethereal Form Only): Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 15 (3d6 + 5) necrotic damage. Targets must also succeed on a DC 18 Constitution saving throw or have their hit point maximum reduced by the damage taken. Only greater restoration, comparable magic, or completing a long rest can undo this reduction.

Tail (Material Form Only): Melee Weapon Attack: +1 to hit, reach 10 ft., 1 target. Hit: 19 (4d6 + 5) bludgeoning damage. 

Enslave (3/day): Bartzabel targets one creature it can see within 30 ft. of it. The target must succeed on a DC 16 Wisdom saving throw or be magically charmed by it until it dies or until it is on a different plane of existence from the target. The charmed target is under Bartzabel's control and can't take reactions, and Bartzabel and the target can communicate telepathically with each other over any distance. 
Whenever the charmed creature takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from Bartzabel.

Void Body: Bartzabel an reduce the damage it takes from a single source to 0. Radiant damage can only be reduced by half.

Bartzabel can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Barzabel regains spent legendary actions at the start of its turn.

Detect. Bartzabel makes a Wisdom (Perception check).

Tail Swipe. Bartzabel makes one tail attack.

Psychic Drain (Costs 2 Actions). One creature charmed by Bartzabel takes 10 (3d6) psychic damage, and Bartzabel regains hit points equal to the damage the creature takes.