Shapechanger. If Relvan 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 its true form.

While in bat form, he can't speak, its 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 it, but nothing he is carrying does. He reverts to its true form if he dies or is rendered unconscious.

While in mist form, Relvan 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.

Magic Resistance. Relvan Ariessus has advantage on saving throws against spells and other magical effects.

Legendary Resistance (3/Day). If Relvan Ariessus fails a saving throw, he can choose to succeed instead.

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

Multiattack. Relvan Ariessus makes three melee attacks, only one of which can be a bite attack.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one willing creature, or a creature that is grappled by Relvan Ariessus, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 7 (2d6) necrotic damage. If the target is humanoid, it must succeed on a DC 15 Charisma saving throw or be charmed by the vampire for 1 minute. While charmed in this way, the target is infatuated with the vampire.

The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Relvan Ariessus 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.

Rapier. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage.

Unarmed Strike. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. Relvan Ariessus can also grapple the target (escape DC 14) if it is a creature and the vampire has a hand free.

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

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

Parry. Relvan Ariessus adds 3 to his AC against one melee attack that would hit it. To do so, Relvan must see the attacker and be wielding a melee weapon.

Relvan 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. Relvan regains spent legendary actions at the start of his turn.

Move. Relvan moves up to his speed without provoking opportunity attacks.

Unarmed Strike. Relvan makes one unarmed strike.

Bite (Costs 2 Actions). Relvan makes one bite attack.