Boulder Snake

Magic Resistance: The Boulder Snake advantage on saving throws against spells and other magical effects.

Poison Immunity: The Snake is immune to poison damage and the poisoned condition.

Anti magic Cone: The Boulder Snakes eyes creates an area of anti magic, as in the anti magic field spell, in a 100-foot cone. At the start of each of its turns, the Snake decides which way the cone faces and whether the cone is active.

Coil. Bonus action on turn: The Snake coils its body and gains +3 AC and an additional +5 to hit on its bite attack. Its movement speed becomes 0 while coiled. The Snake must use another bonus action to become Uncoiled.

Multi Attack: The Boulder Snake can use his action to make one tail slam and one bite attack

Bite. Melee Weapon Attack: +9 to hit, reach 10ft., one target. Hit: 10(3d4+5) piercing damage, and the target must make a DC 16 Constitution saving throw, taking 17 (5d6) poison damage on a failed save, or half as much damage on a successful one. If the target fails the save they're considered Poisoned for the next 24 hours and can repeat the saving throw at the end of each hour ending the effect on a success.

Tail Slam: Melee Weapon Attack: +9 to hit, reach 10ft., one target. Hit: 21(3d10+5) bludgeoning damage, and the target must make a dexterity saving throw(DC 16), or be knocked prone.

Petrifying Gaze: When a creature that can see the Boulder Snake's starts its turn within 30 ft. of the Snake, the Snake can force it to make a DC 16 Constitution saving throw if the Snake isn't Incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly Petrified. Otherwise, a creature that fails the save begins to turn to stone and is Restrained. The Restrained creature must repeat the saving throw at the end of its next turn, becoming Petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the Greater Restoration spell or other magic.

Tag: The Boulder snake can use its opportunity attack to strike out and immobilize it's target. Melee Weapon attack+9, Reach 10 ft., one target. Hit: 20(3d6) poison damage, and the targets speed is reduced to 0.