Amviper

Shapechanger. If the amviper is within sight of a highly reflective surface, it can use its action to polymorph into a Medium swarm of poisonous snakes or a Huge giant constrictor snake, or back into its true form. While in its swarm of poisonous snakes form, the amviper can't speak, its walking speed is 30 feet, and it has a swim speed of 30 feet. It can occupy another creature’s space and vice versa, and it can move through any opening large enough for a Tiny creature. While in giant constrictor snake form, its speed is 30 feet, and it has a swim speed of 30 feet. Its statistics, other than its size and speed, are unchanged. Anything it is wearing transforms with it, but nothing it is carrying does. It reverts to its true form if it dies.

Legendary Resistance (1/day). If the amviper fails a saving throw, it can choose to succeed instead.

Reflective Death. When slain, the amviper’s corpse is pulled towards the nearest highly reflective surface. This surface becomes a portal leading to Deep Barovia and Happy Jack, and the amviper’s corpse appears on the other side of the portal.

Reflective Evasion. The amviper does not reflect an image off of any surface.

Regeneration. The amviper regains 10 hit points at the start of its turn if it has at least 1 hit point and is within sight of a highly reflective surface. This trait doesn't activate on a turn that the amviper takes radiant damage or damage from holy water.

Multiattack (Amviper Form Only). The amviper makes two attacks, only one of which can be an injection attack.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 7 (1d6+3) piercing damage. The target must make a DC 15 Constitution saving throw, taking 14 (4d6) poison damage on a failed save, or half as much damage on a successful one.

Constrict (Giant Constrictor Snake Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 12 (2d8+3) bludgeoning damage. The target is grappled (escape DC 15). Until this grapple
ends, the creature is restrained, and the amviper can’t constrict another target

Unarmed Strike (Vampire Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 7 (1d8+3) bludgeoning damage. Instead of dealing damage, the amviper can grapple the target (escape DC 15).

Injection. Melee Weapon Attack: +7 to hit, reach 5ft., one willing creature, or a creature that is grappled by the amviper, incapacitated, or restrained. Hit: 6 (1d6+3) piercing damage. The target must make a DC 15 Constitution saving throw, transforming into a swarm of poisonous snakes on a failed save. While in swarm of poisonous snakes
form, the target can’t speak, its walking speed is 30 feet, and it has a swim speed of 30 feet. It can occupy another creature’s space and vice versa, and it can move through any opening large enough for a Tiny snake. The target also can’t regain hit points or gain temporary hit points.