Gnoll Vampire

The vampire has advantage on Wisdom (Perception) checks that rely on smell.

When it reduces a creature to 0 hit points with a melee attack on its turn, the vampire can take a bonus action to move up to half its speed and make a bite attack.

The vampire regains 10 hit points at the start of its turn if it has at least 1 hit point and isn't in sunlight or running water. If the vampire takes Radiant damage or damage from holy water, this trait doesn't function at the start of the vampire's next turn.

If the vampire isn't in sunlight, it can use its action to polymorph into a Large hyena or a Medium cloud of mist, or back into its true form.
While in hyena form, the vampire can't speak, and its walking speed is 50 feet. Its statistics, other than its size and speed, are unchanged. Anything it is wearing transforms with it, but nothing it is carrying does. It reverts to its true form if it dies.
While in mist form, the vampire can't take any actions, speak, or manipulate objects. it is weightless, has a flying speed of 20 feet, can hover, and can enter a hostile creature's space and stop there. In addition, if air can pass through a space, the mist can do so without squeezing, and it can't pass through water. It has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to all nonmagical damage, except the damage it takes from sunlight.

The vampire doesn't require air.

The vampire has the following flaws:
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Enraged by Celestial
If it hears words of Celestial spoken, the vampire must try to attack the source of those spoken words on its next turn. If these words come from multiple sources and from opposite directions, the vampire is Restrained. Otherwise, it moves to attack what it perceives to be the closest source.
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Repulsed by Perfume
The vampire has disadvantage on melee attack rolls made against any creature wearing perfume or carrying an open container of it.
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Stake to the Heart
If a piercing weapon made of wood is driven into the vampire's heart while the vampire is Incapacitated in its resting place, the vampire is Paralyzed until the stake is removed.
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Sunlight Hypersensitivity
The vampire takes 20 Radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

The vampire makes two attacks: one with its bite and one with its claws.

Melee Weapon Attack +8 to hit, reach 5 ft., one creature. 12 (2d6 + 5) Piercing damage plus 9 (2d8) Necrotic damage. The target's hit point maximum is reduced by an amount equal to the Necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. the target dies if its hit point maximum is reduced to 0.

Melee Weapon Attack +8 to hit, reach 5 ft., one target. 10 (2d4 + 5) Slashing damage.

The vampire emits a bone-chilling cackle. Each creature of the vampire's choice that is within 120 feet of the vampire and can hear its cackle must succeed on a DC 15 Wisdom saving throw or become Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the vampire's Frightful Cackle for the next 24 hours.

The vampire targets one humanoid it can see within 30 feet of it. If the target can see the vampire, the target must succeed on a DC 15 Constitution saving throw against this magic or be Poisoned for 24 hours. A creature whose saving throw is successful is immune to this vampire's Sickening Gaze for 24 hours.