Legendary Resistance (?/Day). If the vampire fails a saving throw, it can choose to succeed instead.
Shapechanger. use its action to polymorph into a Medium cloud of mist, or back into its true form.
Mist form: can't take any actions, speak, or manipulate objects. it is weightless, has a flying speed of 20 ft (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 Str, Dex, and Con saving throws, and it is immune to all nonmagical dmg, except sunlight.
Keen Smell. has adv on smell Perception checks.
Regeneration. Recover 10 hp at the start of its turn if it has at least 1 hp and isn't in sunlight or running water. If takes radiant dmg, this trait doesn't function at the start of it's next turn.
Unusual Nature. The vampire doesn't require air.
Vampire Weaknesses
Multiattack. two attacks, only one can be a bite attack.
Bite. +8 to hit, Hit: 12 (2d6+5) piercing dmg plus 9 (2d8) necrotic dmg. The target's hp maximum is reduced by an amount equal to the necrotic dmg taken, and the vampire regains hp 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.
Claws. +8 to hit, Hit: 10 (2d4+5) slashing dmg.
Frightful Hiss. Each creature of the vampire's choice that is within 120 feet of the vampire and can hear 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.