Vampyr, Lord of Blood

Regeneration. Vampyr regains 30 hit points at the start of his turn if he has at least 1 hit point and isn't in running water or sunlight. If he takes radiant damage or damage from holy water, this trait doesn't function at the start of his next turn  

Composite Nightmare. Given form by the Ritual of Amber, Vampyr is a composite of its true form and the form of the vessel into which it has been bound. Vampyr is considered both a Fiend and the creature type of the vessel. Vampyr can be targeted by any effect that targets one of its two types but has advantage on any saving throw against any effect that targets only one of its two types.

Drink. As a bonus action, Vampyr can drain the lifeforce from all creatures connected to it by tendrils. Each creature takes 22 (4d6+8) necrotic damage, its hit point maximum is reduced by an amount equal to the necrotic damage taken, and Vampyr regains hit points equal to the total damage dealt. 

Legendary Resistance (3/day). If the Vampyr fails a saving throw, it can choose to succeed instead.

Multiattack. Vampyr uses its Call to Blood attack three times. It can replace two of these attacks with a Slam attack.

Slam. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 40 (6d10 + 10) bludgeoning damage. 

Call to Blood. Ranged Weapon Attack: +15 to hit, range 60 ft., one target. Hit: 25 (3d10 + 9) slashing damage plus 12 (3d8) necrotic damage. The target must make a DC 22 Constitution saving throw. On a failure, a fleshy tendril of congealed blood forms, connecting the target to Vampyr. A target makes this save with disadvantage if it is at less than full health. A tendrilled target cannot move further than 80 feet away from Vampyr unless Vampyr allows it.
A target can only be affected by a single tendril at a time. The tendril has an AC of 10, 18 HP, and the same damage resistances and vulnerabilities as Vampyr. A creature can use its action to attempt a Strength (Athletics) check against DC 20 to pull a tendril free, dealing 15 (5d6) piercing damage to the tendrilled target on a success.

Eruption. Vampyr rips all tendrils free of their hosts. Each tendrilled creature must take a DC 22 Constitution saving throw or take 22(4d6 +8) piercing damage and be Stunned until the end of Vampyr’s next turn. On a successful saving throw, the creature takes half damage and is not Stunned.

Vampyr can take 4 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. Vampyr regains spent legendary actions at the start of its turn.

Call to Blood. Vampyr uses its Call to Blood attack.
Bloody Mist. Vampyr briefly erupts into a cloud of blood, rushing across the battlefield. Vampyr moves up to its speed without provoking attacks of opportunity.
Eruption (Costs 3 Actions). Vampyr uses its Eruption action.