Rules of the Heads :

Weak points: Each Abomination have threes heads/mouths, when their mouths are open it's exposes a bright blue light, that's the weak points of the beast. Roll a 1d4 at the beginning of combat & every other rounds to judge which mouth will be open (Below this text). If a that weak point is being attack, roll a DC 17 Constitution Saving Throw. If failed, that head is pop and doubles the damages It was taking before.

1. Left Mouth is open.
2. Middle Mouth is open.
3. Right Mouth is open.
4. All Mouths are Close.  

Damage effect: At the Max of heads (3), The Abomination Bite and Lightning Breath damages are at the current "Max". Each time it loses a Head, the damages of Bite and Lightning Breath are decreased by what is listed below:

• If three heads still stands:
  • Lightning Breath deals 66 (12d10) Lightning damage
  • Bite deals 18 (6d6) Piercing damage
• If two heads stands:
  • Lightning Breath deals 40 (8d10) Lightning damage
  • Bite deals 12 (4d6) Piercing damage
• If only one head stands:
  • Lightning Breath deals 20 (4d10) Lightning damage
  • Bite deals 6 (2d6) Piercing damage

Multiattack. The Abomination makes one Ram attack or one Bite attack, and one Tail whip attack.

Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 18 (6d6) Piercing damage  (or less base on the Rule of Heads  trait).

Tail whip. Melee Attack Roll: +7, reach 5 ft. Hit: 7 (1d6 + 4) Bludgeoning damage.

Ram. Melee Attack Roll: +7, reach 5 ft. Hit: 10 (1d12 + 4) Bludgeoning damage. If the target is a Medium or smaller creature, it has the Prone condition.

Lightning Breath (Recharge 5–6). Abomination exhales a line of lightning that is 20 feet long and 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 66 (12d10) lightning damage (or less base on the Rule of Head  trait) on a failed save, or half as much damage on a successful one