Shapechanger. If Jander isn't in sunlight or running water, he can use his bonus action to polymorph into a Tiny bat, a Medium cloud of silver mist, a Medium golden wolf, or back into his true form.

While in bat form, he can't speak, his walking speed is 5 feet, and he has a flying speed of 30 feet. His statistics, other than its size and speed, are unchanged. Anything he is wearing transforms with him, but nothing he is carrying does. He reverts to its true form if he drops to 0 hit points.

While in wolf form, he can't speak and his walking speed is 40 feet. His statistics, other than size and speed are unchanged. Anything he is wearing transforms with him, but nothing he is carrying does. He reverts to its true form if he drops to 0 hit points.

While in mist form, Jander 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. He has advantage on Strength, Dexterity, and Constitution saving throws, and he is immune to all nonmagical damage, except the damage it takes from sunlight.

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

Dead and Loving it.  When Jander drops to 0 hit points, the Mists interfere preventing him from transforming into mist. He must be taken to a safe space to regenerate. Failure to do so will result in the Mists taking Jander and moving him to a random safe location on the plane to regenerate. Until he can begin regenerating he is incapacitated.

Regeneration. Jander 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 Jander takes radiant damage or damage from holy water, this trait doesn't function at the start of the his next turn.

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

Dueling. When Jander is wielding a melee weapon in one hand and no other weapons, he gains a +2 bonus to damage rolls with that weapon.

Black Thumb. Any plant that Jander touches withers and dies.

Vampire Weaknesses. Jander has the following flaws:

Forbiddance. He can't enter a residence without an invitation from one of the occupants.

Harmed by Running Water. He takes 20 acid damage if it ends its turn in running water.

Stake to the Heart. If a piercing weapon made of wood is driven into the Jander's heart while he is incapacitated he is paralyzed until the stake is removed.

Sunlight Hypersensitivity. Jander takes 20 radiant damage when he starts his turn in sunlight. While in sunlight, he has disadvantage on attack rolls and ability checks.

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

+2 Shortsword. Melee Weapon Attack: + 10 to hit, reach 5ft., one target. Hit: 1d6 + 8 piercing.

+1 Dagger.  Melee Weapon Attack: + 9 to hit, reach 5ft., one target. Hit 1d4 + 7 piercing.

Unarmed Strike (Vampire Form Only). Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 8 (1d8 + 4) bludgeoning damage. Instead of dealing damage, Jander can grapple the target (escape DC 18).

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 Jander, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and Jander 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. 

Charm. Jander targets one humanoid he can see within 30 feet of it. If the target can see Jander, the target must succeed on a DC 17 Wisdom saving throw against this or be charmed by him. The charmed target regards him as a trusted friend to be heeded and protected. Although the target isn't under Jander's control, it takes his requests or actions in the most favorable way it can, and it is a willing target for Jander's bite attack.

Each time Jander 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 Jander is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

Children of the Night (1/Day). As long as the beasts are not actively under Strahd's control Jander can magically call 2d4 swarms of bats or rats (swarm of bats, swarm of rats). While outdoors, the vampire can call 3d6 wolves (wolf) instead. The called creatures arrive in 1d4 rounds, acting as allies of Jander and obeying his commands. The beasts remain for 1 hour, until the vampire dies, Strahd over powers his connection, or until the vampire dismisses them as a bonus action.