Shapechanger. If Yao isn’t in sunlight or running water, he can use his action to polymorph into a Tiny bat or a Medium cloud of mist, or back into his true form.

While in bat form, Yao can’t speak, his walking speed is 5 feet, and he has a flying speed of 30 feet. His statistics, other than his size and speed, are unchanged. Anything he is wearing transforms with him, but nothing he is carrying does. Yao reverts to his true form if he dies.

While in mist form, Yao can’t take any actions, speak, or manipulate objects. He 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.

Legendary Resistance (1/Day). If Yao fails a saving throw, it can choose to succeed instead.

Misty Escape. When Yao drops to 0 hit points outside his resting place, Yao transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that he isn’t in sunlight or running water. If he can’t transform, he is destroyed.

While Yao has 0 hit points in mist form, he can’t revert to his vampire form, and Yao must reach his resting place within 2 hours or be destroyed. Once in his resting place, he reverts to his vampire form. Yao is then paralyzed until he regains at least 1 hit point. After spending 1 hour in his resting place with 0 hit points, Yao regains 1 hit point.

Regeneration. Yao regains 20 hit points at the start of his turn if he has at least 1 hit point and isn’t in sunlight or running water. If Yao takes radiant damage or damage from holy water, this trait doesn’t function at the start of his next turn.

Spider Climb. Yao can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Vampire Weaknesses.  Yao has the following flaws:

Forbiddance. Yao can’t enter a residence without an invitation from one of the occupants.

Harmed by Running Water. Yao takes 20 acid damage if he ends his turn in running water.

Stake to the Heart. If a piercing weapon made of wood carved within the last 24 hours is driven into Yao’s heart while he is incapacitated in his resting place, Yao is paralyzed until the stake is removed.

Time Capsule. When Yao is awoken in his lair, he can choose to suppress one of his vampiric weaknesses for the next 8 hours.

Special Equipment. Yao wields a +3 silver longsword and wears +3 plate armor that grants him fire resistance.

Multiattack. (Vampire Form Only). Yao makes three attacks, only one of which can be a bite attack.

Longsword (Vampire Form Only). Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 19 (4d8 + 9) slashing damage.

Bite. (Bat or Vampire Form Only). Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by Yao, incapacitated, or restrained. Hit: 9 (1d6 + 6) piercing damage plus 14 (4d6) necrotic damage and the target has disadvantage against Yao's charm for 1 minute. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and Yao regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under Yao's control.

Charm. Yao targets one humanoid he can see within 60 feet of him. If the target can see Yao, the target must succeed on a DC 17 Wisdom saving throw against this magic or be charmed by Yao. The charmed target regards Yao as a trusted friend to be heeded and protected. Although the target isn't under the Yao’s control, it takes the Yao's requests or actions in the most favorable way it can, and it is a willing target for Yao's bite attack.

Each time Yao or his companions do anything harmful to the target, it can repeat  the saving throw, ending the effect on itself on a success. Otherwise, the effect lasts 24 hours or until Yao is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

Hell Step (3/Day). As a bonus action, Yao can teleport up to 30 feet to a point he can see. All creatures within 5 feet of the point from which he teleported suffer 7 (2d6) fire damage.

Child of the Night (1/Day). Yao magically calls Tianshi, his angelic artificial daughter.  She arrives to assist her father within 1d6 turns.