Corruption

Shapechanger. If the vampire isn’t in sunlight or running water, it can use its action to polymorph into a Medium cloud of mist, or back into its true form.

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 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.

Alien Mind. If a creature tries to read Corruption’s thoughts or deals psychic damage to it, that creature must succeed on a DC 23 Intelligence saving throw or be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Legendary Resistance (5/Day). If the vampire fails a saving throw, it can choose to succeed instead.

Regeneration. The vampire regains 60 hit points at the start of its turn if it has at least 1 hit point and isn’t in sunlight or running water.

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

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

Inhuman Speed. Creatures cannot react to Corruption's actions, movements, or reactions unless their passive Insight is 21 or higher. On their turn, a creature can use their action to make a Wisdom (Insight) contested by Corruption's Charisma (Deception) check. On a success, that creature can react to Corruption as normal until Corruption leaves their line of sight. Additionally, at the start of each combat Corruption can move with a burst of speed, causing each creature within 60 feet of him to make a DC 27 Constitution saving throw or become Deafened. Corruption can take a total of three reactions on each round of combat.

Unholy Strength. Corruption's claw attacks puncture armor and rend flesh. When corruption's attack rolls exceed an armored opponent's armor class by 10 or more, that piece of armor takes a cumulative -1 penalty to armor class. The armor is destroyed when the creature's armor class is reduced to the same as their unarmored armor class.

Immortality in Undeath. To truly kill and Elder Vampire, the following steps must be performed:
0. The vampire must be reduced to zero hit points whilst the following effects occur:
1. The vampire must be drained by one of its spawn whilst within its lair
2. The vampire must have its heart cut out and devoured by a full vampire
3. The vampire must be fully submerged and drowned in holy water soaked in celestial ichor
4. The vampire must be restrained by silver chains and staked through the heart with a wooden stake that had killed the wielder's loved one.
5. The vampire must drink the blood of an unwilling good-aligned humanoid
6. The vampire must be dismembered and each severed limb must be coated in salt and iron
7. The vampire's body must be set alight with magical flames
8. The vampire must be decapitated with a garotte made from its own entrails
9. The vampire must be hit with the 'Dismissal' property of the Dispel Evil and Good spell and fail the saving throw.
10. Corruption's torso must be impaled with three spears and held aloft in the air, exposed to open sunlight. The first spear must be made of Adamantine, the second must be made of Mythril and the final must be made of Silver.

Multiattack. The vampire makes three attacks, two with its claws and one with its bite.

Claw. Melee Weapon Attack: +19 to hit, reach 5 ft., one creature. Hit: 16 (1d12 + 10) slashing damage. The vampire can grapple the target as part of this attack (escape DC 20). The creature must succeed a DC 24 Intelligence saving throw or be stunned until the grapple ends.

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained. Hit: 13 (1d6 + 10) piercing damage plus 42 (12d6) 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 Greater Restoration is cast on the target at 9th level. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way rises as a vampire spawn under the vampire’s control at the start of the Vampire's next turn.

Charm. 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 22 Wisdom saving throw against this magic or be charmed by the vampire. The charmed target regards the vampire as a trusted friend to be heeded and protected. Although the target isn’t under the vampire’s control, it takes the vampire’s requests or actions in the most favorable way it can, and it is a willing target for the vampire’s bite attack.

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

Devour Brain. Melee Weapon Attack: +19 to hit, reach 5ft, one incapacitated humanoid grappled by Corruption. Hit:  99 (18d10) Piercing Damage. If this damage reduces a target to zero hit points, Corruption devours the target's brain, killing them instantly and restoring one legendary resistance.

Mind Blast (Recharge 5-6). All creatures in a 120 feet cone must succeed on a DC 22 Intelligence saving throw or take 44 (8d10) psychic damage and be stunned for one minute. A stunned creature can repeat this save at the end of each of their turns.

Corrupt Thought (1/Day). Corruption can disturb and corrupt a single creature's thoughts that it shares the same plane of existence, that creature must succeed a DC 25 Charisma saving throw or be inflicted with a madness of corruption. A creature targeted with a greater restoration spell can repeat this saving throw.

Dismember. When a creature misses with a melee attack roll Corruption can use its reaction to make a claw attack against the creature. On a hit, instead of dealing damage Corruption can attempt to rip off the target's limb. That target must succeed a DC 27 Strength saving throw or have the limb used to make the attack be ripped off.

Deflect. When Corruption is targeted with a ranged attack it can use its reaction to try catch the arrow out of the air, reducing the damage dealt by 1d10+25.

The vampire can take 5 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. The vampire regains spent legendary actions at the start of its turn.

Move. The vampire moves up to its speed without provoking opportunity attacks. It can instead try to break an effect that is restricting its movement, such as being grappled or restrained.

Claw. The vampire makes one unarmed strike.

Mind Blast (Costs 2 Actions). Corruption makes one mind blast attack.

Bite (Costs 2 Actions). The vampire makes one bite attack.

Devour (Costs 3 Actions). The vampire makes one Devour Brain attack.

Sanguinate (Costs 5 Actions). Corruption attempts to bite a creature, ignoring whether or not they are currently grappled or willing. If that bite attack hits that creature must make a DC 27 Consitution saving throw or be completely drained of their blood. A creature drained of their blood is reduced to zero hit points and makes all of their death saves at the start of their next turn. If that creature dies, Corruption regains a number of hit points equal to the hit points the target had.