Shapechanger. If Acoly isn’t in rain or running water, he can use his action to polymorph into a hell-hound, or a miasma of fire called flamecloud, or back into his true form. 

While in hell-hound form, the vampire can’t speak, it has a movement speed of 60. Its statistics, other than its size and speed, are unchanged. 

While in flamecloud form, Acoly can’t take any actions, speak, or manipulate objects. He is weightless, has a flying speed of 30 feet, but must remain moving each turn or settle on a flammable object, which includes a creature wearing clothing. If he settles on a creature wearing dry clothing, they must make a DEX 16 save or take 2d10 fire damage if they start their turn within the flamecloud. 

If Acoly is impeded by water or solid objects or cannot move on his turn, he must make a DC 18 CON saving throw or return to humanoid form.

Flamecloud has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to all nonmagical damage, except damage it takes from water or ice. 

Regeneration. Acoly regains 10 hit points at the start of his turn if he has at least 1 hit point and isn’t touching water. 

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

Fiery Escape. When he drops to 0 hit points, he transforms into flamecloud shape with 6 temporary HP instead of falling unconscious (provided that he isn't standing in rain or running water). His movement speed in this form increases to 60 feet. Until he regains actual HP, he cannot use any abilities or traits such as regeneration, legendary actions, etc.

If Acoly cannot transform into the flamecloud (I.E. because of heavy rain) he is destroyed.

Multiattack: Lord Acoly makes three leechblade attacks.

Leechblade. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: (1d8 slashing +4 necrotic damage)

If Acoly deals more than 20 damage to a single creature, he regains 1d8 HP. If a leechblade attack reduces a creature to 0 hp, Acoly regains 3d8 HP.  

Alpha Pater: Acoly breathes fire and creates a single [creature]hellhound[/creature] .  

Spellcasting.  Acoly is a 4th-level spellcaster (Warlock lvl 10). His spellcasting ability is Charisma (spell save DC 14, +5 to hit with spell attacks). He has only 3 pact magic slots, each cast at 4th level.

Acoly can cast eldritch blast at will, making two blasts with +4 modifier to damage; each blast may push the target 10 feet if he chooses. 

hellish rebuke (4d10 fire damage), blink

Acoly can take 2 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. He regains spent legendary actions at the start of his turn.

Fiendish Rush.  Lord Acoly moves too quickly for the eye to follow, speeding up to 30 feet, immune to opportunity attacks. Once he reaches target, he can make one leechblade attack; if the attack lands, the creature must make a DC 12 STR check or be knocked 15 feet backwards. 

If he is currently using the blink spell, he returns to the material plane for the duration of fiendish rush and when his attacks have concluded, he rolls (as if he had ended a turn) to see if he returns to the immaterial plane. 

Alpha Cannibal: If Lord Acoly is within 50 feet of a hellhound, he makes a tremendous inhalation and rolls a spell attack. If he succeeds, he absorbs its life force; the creature dies and he regains 1/2 of its HP max. 

Potshot: Acoly uses eldritch blast.