Amphibious. The horned serpent can breathe air and water.

Corpse Cough. Those who fail a saving throw against the horned serpent's Gem 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, wet cough. The infected creature must succeed on a DC 17 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 a blood relative before the disease is cured, the relative must succeed on a DC 17 Constitution saving throw or also become infected with the disease. The blood relative 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 horned serpent's 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 horned serpent is immune to scrying and to any effect that would sense its emotions, read its thoughts, or detect its location.