Icy Walk. The chromatic abomination can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

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 for 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 Attacks: +10 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 6) piercing damage plus 6 (2d6) cold or fire damage (Its choice).

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 6) slashing damage.

Cold Breath (Recharge 5-6). The chromatic abomination exhales an icy blast in a 30-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 40 (10d8) cold damage on a failed save, or half as much on a successful one. 

Fire Breath (Recharge 5-6). The chromatic abomination exhales fire in a 30-foot cone. Each creature in that area must make a DC 17 Dexterity saving throw, taking 48 (16d6) fire damage on a failed save, or half as much on a successful one.