Crystal Basilisk

Petrifying 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 15 Constitution saving throw if the basilisk isn't incapacitated On a failed save, the creature magically begins to turn to crystal and is restrained.  It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

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.

Shredding Defenses: A creature within 5 feet of the Crystal Basilisk that hits it with an attack, takes 4 (1d6) piercing damage. This damage is considered magical for the purposes of bypassing defenses.

Death Burst: When the Crystal Basilisk dies, it explodes in a burst of stinging energy. Each creature within 30 feet of it must make a DC 15 Dexterity saving throw, taking 10 (3d6) acid, and 10 (3d6) piercing damage on a failed save, or half as much damage on a successful one.

Tunneler. The Basilisk can burrow through solid rock at half its burrow speed and leaves a 5-foot-diameter tunnel in its wake.

Multiattack. The basilisk makes two attacks: one with its bite and one with its tail.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 18 (4d6 + 5) piercing damage plus 10 (3d6) poison damage.

Tail. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 20 (3d10 + 5) piercing damage. If the target is a Large or smaller creature, it is grappled (escape DC 15).