Stench.

Any creature other than a catoblepas that starts its turn within 10 feet of the catoblepas must succeed on a DC 16 Constitution saving throw or be poisoned until the start of the creature's next turn. On a successful saving throw, the creature is immune to the Stench of any catoblepas for 1 hour.

Actions

Tail.

Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 21 (5d6 + 4) bludgeoning damage, and the target must succeed on a DC 16 Constitution saving throw or be stunned until the start of the catoblepas's next turn.

Death Ray (Recharge 5–6).

The catoblepas targets one creature it can see within 30 feet of it. The target must make a DC 16 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one. If the saving throw fails by 5 or more, the target instead takes 64 necrotic damage. The target dies if reduced to 0 hit points by this ray.

Environment: Swamp

Catoblepas