Multiattack

The horned serpent makes one gore attack and one bite attack.

Gore

Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 19 (4d6 + 5) piercing damage.

Bite

Melee Weapon Attack: +10 to hit, reach 10 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 horned serpent's gem flashes, bathing a 30-foot cone in iridescent light. Each creature in the area must make a DC 17 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.