Bloodviper

 Life Drain. If somebody ends their turn in the bloodviper's constrict grapple it drains the person for an extra (2d6+4) necrotic damage and the Bloodviper regains that much health

Multiattack. The Bloodviper makes two attacks

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the Bloodviper, incapacitated, or restrained. Hit: 6 (1d6 + 3) piercing damage plus 7 (2d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Bloodviper 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.

Constrict. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 13 (2d8 + 4) bludgeoning damage, and the target is grappled (escape DC 16). Until this grapple ends, the creature is restrained, and the snake can't constrict another target.

Rattle. The Bloodviper shakes the end of its tail, emitting a rattling sound that can be heard from up to 30 feet. Each creature that can hear the rattle must succeed on a DC 8 Wisdom saving throw or be frightened of the rattlesnake for 1 minute. A creature can end the condition by spending at least 20 feet of movement to get away from the snake.