• Amphibious. The adder can breathe air and water.
• Corpse Cough. Those who fail a saving throw against the adders Gaze or bite attack become infected with the corpse cough disease. The infected creature can’t benefit from short or long rests due to a constant, cough. The infected creature must succeed on a DC 18 Constitution saving throw each day or take 18 (4d8) necrotic damage. The target’s hp maximum is reduced by an amount equal to the necrotic damage taken. The target dies if this effect reduces its hp maximum to 0. The reduction lasts until the target is cured of the disease with a greater restoration spell or similar magic. If the infected creature comes into physical contact with another individual for 24 hours the individual must succeed on a DC 17 Constitution saving throw or also become infected with the disease. The individual is afflicted with a constant, wet cough within hours of infection, but the disease’s full effects don’t manifest until 1d4 days later. Corpse cough is so named due to the smell of the cough as the infected creature’s lungs become necrotic.
• Poisonous Aura. At the start of each of the adders turns, each creature within 20 feet of it must make a DC 17 Constitution saving throw, taking 18 (4d8) poison damage on a failed save, or half as much damage on a successful one.
• Shielded Mind. The adder is immune to scrying and to any effect that would sense its emotions, read its thoughts, or detect its location.
• Legendary Resistance (3/Day). If the adder fails a saving throw, it can choose to succeed instead.

• Multiattack. The Adder makes two gore attacks and one bite attack and fearful prescense.
• Gore. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 19 (4d6 + 5) piercing damage.
• Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 21 (3d10 + 5) piercing damage, and the target must succeed on a DC 17 Constitution saving throw or become infected with the corpse cough disease (see the Corpse Cough trait).
• Gem Gaze (Recharge 5–6). The Adders gem flashes, bathing a 30-foot cone in iridescent light. Each creature in the area must make a DC 18 Constitution saving throw. On a failed save, a creature takes 35 (10d6) radiant damage and is infected with the corpse cough disease (see the Corpse Cough trait). On a success, a creature takes half the damage and isn’t infected with the disease. Gem Gaze has no effect on constructs and undead.
• Frightful Presence. Each creature of the adders choice that is within 120 feet of the adder and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the adders Frightful Presence for the next 24 hours.
• Tail. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

The adder can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The adder regains spent legendary actions at the start of its turn.

Detect. The adder makes a Wisdom (Perception) check.

Tail Attack. The adder makes a tail attack.

body Attack (Costs 2 Actions). The adder swings its body. Each creature within 20 feet of the adder must succeed on a DC 19 Dexterity saving throw or take 15 (2d8 + 6) bludgeoning damage and be knocked prone.