Ancient Basilisk

Deathly Gaze. If a creature starts its turn within 30 feet of the basilisk and the two of them can see each other, the basilisk can force the creature to make a DC 30 Constitution saving throw if the basilisk isn't incapacitated or blinded. On a failed save, the creature is reduced to one (1) hit point.

A creature that isn't surprised can avert its eyes to avoid the saving throw at the start of its turn. If it does so, it can't see the basilisk until the start of its next turn, when it can avert its eyes again.

If it looks at the basilisk in the meantime, it must immediately make the save. If the basilisk sees its reflection within 30 feet of it in bright light, it mistakes itself for a rival and targets itself with its gaze, taking 20 damage and becoming blinded.

Withering Bite. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 20 (4d6 + 3) piercing damage plus 20 (2d10) poison damage per turn for 1d4 turn(s).

Tail Smash. Melee Weapon Attack: +10 to hit, reach 20 ft., 1d4 target(s). Hit: 12 (2d6).

Legendary Resistance (3/Day). If the basilisk fails a saving throw, it can choose to succeed instead.

Corrosive Breath. Breath Attack: The basilisk exhales acid in a 30-­foot cone that is 5 feet wide. Each creature in that cone must make a DC 18 Dexterity saving throw, taking 30 (3d10) acid damage and -10 Armor Class on a failed save, or half as much damage and -5 AC on a successful one. The Armor debuffs last for three turns, and cannot be repaired.