Vampire Swarm

Regeneration. The swarm regains 10 hit points at the start of its turn if it has at least 1 hit point and isn't in sunlight or running water. If it takes radiant damage or damage from holy water, this trait doesn't function at the start of the vampire's next turn.

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

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a medium creature. 

Vampire Weaknesses. The vampire swarm has the following flaws:

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

Harmed by Running Water. The vampire swarm takes 20 acid damage when it ends its turn in running water.

Sunlight Hypersensitivity. The vampire takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

Multiattack. The swarm makes two claw attacks. If  both attacks hit a Medium or smaller target, the target is grappled.(escape DC 13), and the swarm uses its Engulf attack on it.

Claws. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 8 (2d4 + 3) slashing damage, or 5(1d4+3) if the swarm is at half or less hit points.

Engulf:  The  Vampire Swarm engulfs a Medium or smaller creature is grappled by it. The engulfed target is blinded, restrained, and unable to breathe, and it must succeed on a DC 13 Constitution saving throw at the start of each of the mound's turns or take 6 (1d6 + 3) piercing damage plus 7 (2d6) necrotic damage, or 3(1d6)Piercing damage plus 4(1d6) necrotic damage if the swarm has half its hitpoints or less.  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 target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A target who has died in this way joins the Swarm, and is not recoverable.  If the Vampire Swarm moves, it takes the Engulfed creature with it.