Basilisk

Basilisk CR: 3

Medium monstrosity, any

Armor Class: 12

Hit Points: 8d8+16

Speed: 20 ft

STR

16 +3

DEX

8 -1

CON

15 +2

INT

2 -4

WIS

8 -1

CHA

7 -2

Damage Vulnerabilities: Weasels

Damage Immunities: Poison, Acid

Senses: Darkvision 60 ft., Passive Perception 9

Languages: None

Challenge Rating: 3

Toxic Cloud. Any creature that starts its turn within 10 feet of the basilisk must succeed on a DC 14 Constitution saving throw or be poisoned until the start of its next turn.
Venomous Gaze. If a non-basilisk creature starts its turn within 30 ft. of the basilisk and the two of them can see each other, the basilisk can force the creature to make a DC 12 Constitution saving throw if the basilisk isn't incapacitated. On a failed save, the creature becomes infected with the basilisk's venom. Until cured, the venom deals 36 (8d8) poison damage to the creature at the end of its turn. 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.
Weakness to Weasels. The odour of a living or dead weasel is toxic to the basilisk.

Actions

Acidic Touch. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) poison damage plus 7 (2d6) acid damage.
Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage plus 7 (2d6) poison damage.

The lair of a basilisk is easily identifiable by dead vegetation. The basilisk's skin produces enough venom that plants in the area don't stand a ghost of a chance.

"Anyone who sees the eyes of a basilisk serpent (Basilisci serpentis) dies immediately... Its touch and even its breath scorch grass, kill bushes and burst rocks. Its poison is so deadly that once when a man on a horse speared a basilisk, the venom travelled up the spear and killed not only the man, but also the horse. A weasel can kill a basilisk; the serpent is thrown into a hole where a weasel lives, and the stench of the weasel kills the basilisk at the same time as the basilisk kills the weasel." —Pliny the Elder

The basilisk is the most venomous creature known to Theros. Its touch is venomous, its breath is venomous, even eye contact with it is venomous. Thankfully, its eye venom can be avoided by using a blindfold, though that doesn't exactly help avoid its touch or breath. Any part of a basilisk might be profitable to sell. Basilisk blood is a pricey commodity due to the obvious challenges in obtaining it. Pharika claims that it can be used to make antidotes, but few have even been able to obtain it to experiment with.