Amphibious. The chromatic abomination can breathe air and water.

Multiple Heads. The chromatic abomination has two heads. While it has more than one head, the chromatic abomination has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Reactive Heads. For each head the chromatic abomination has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the chromatic abomination sleeps, the other head is awake.

Multiattack. The chromatic abomination makes four attacks: two bite attacks and two claw attacks.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 5) piercing damage plus 6 (2d6) lightning or poison damage (Its choice).

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 5) slashing damage. 

Lightning Breath (Recharge 5-6). The chromatic abomination exhales lightning in a 60-foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 50 (10d10) lightning damage on a failed save, or half as much damage on a successful one.

Poison Breath (Recharge 5-6). The chromatic abomination exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 16 Constitution saving throw, taking 36 (12d6) poison damage on a failed save, or half as much on a successful one.