Blighting Aura. The viper is surrounded by an aura of necrotic elemental magic. Foliage withers and trees smolder with purple smoke. A creature that starts its turn within 30 feet of the viper must succeed a DC 15 Constitution saving throw. The target takes 8d8 necrotic damage on a failed save, or half as much damage on a successful one. This aura has no effect on undead or constructs. Plant creatures and magical plants make the saving throw with disadvantage, and the aura deals maximum damage to it. Nonmagical plants that aren't a creature, such as a tree or shrub, don't make a saving throw, it simply withers and dies. 

The rottenmouth viper can use a bonus action to expand the radius of this aura by 5 feet.

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.