Detect Good. The Abyssal basilisk can detect fey and celestial creatures within 60 feet as an ongoing effect.

Foul Breath. Constitution Saving Throw: DC 13, any creature that starts its turn in or enters a 5-foot Emanation originating from the Abyssal basilisk. Failure: The target has the Poisoned condition until the start of its next turn. 

Magic Resistance. The Abyssal basilisk has Advantage on saving throws against spells and other magical effects.

Petrifying Gaze. Constitution saving throw: DC 12, each creature starting its turn within 30 feet of the basilisk and the two of them can see each other. Failure: the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. Success: effect ends. 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 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.

Multiattack. The Abyssal basilisk makes one Bite attack and two Claw attacks.

Bite. Melee Attack Roll:  +8 to hit, reach 5 ft. Hit: 17 (3d8 + 4) piercing damage plus 10 (3d6) poison damage.

Claw. Melee Attack Roll:  +8 to hit, reach 5 ft. Hit: 13 (2d8 + 4) slashing damage