Undead Fortitude. If damage reduces the rattlesnake to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the rattlesnake drops to 1 hit point instead.
Skeletal Coils. The rattlesnake can occupy another creature’s space and grapple them. The grappled target is restrained and takes 1d6 bludgeoning damage at the start of each of its turns until the grapple ends (escape DC 13).
Actions
Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 7 (1d8 + 3) piercing damage plus 9 (2d8) necrotic damage. The target must succeed on a DC 13 Constitution saving throw or be poisoned for 1 minute.
Tail Whip. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 8 (1d10 + 3) bludgeoning damage.
Necrotic Spit(Recharge 6). The rattlesnake spits necrotic venom in a 15-foot line that is 5 feet wide. Each creature in that line must make a DC 13 Dexterity saving throw, taking 14 (4d6) necrotic damage on a failed save, or half as much on a successful one.
Bonus Actions
Rattling Dread(Recharge 5–6). As a bonus action, the rattlesnake shakes its bony rattle, creating an eerie, necrotic sound. All creatures within 20 feet of it must succeed on a DC 13 Wisdom saving throw or be frightened until the end of the rattlesnake’s next turn.