Charrogg Behemoth

Enraged. When the Charrog falls to half its hit points, it can use its reaction to enter an Enraged state. While Enraged, the Charrog gains resistance to all damage from non-magical weapons, and has advantage on saving throws against being frightened or charmed. It also gains +1 modifier to attack rolls, damage, and Strength, Constitution, and Dexterity ability checks and saving throws.

Poisoning Resistance. The Charrog has advantage on saving throws against being poisoned.

Multiattack. The Charrog makes one Bite attack and one Tail or Headbutt attack. If the Headbutt knocks the target prone, the Charrog can make a Clawed Stomp attack against it.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage + 2 (1d4) fire damage. 

Headbutt. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage and the target must succeed on a DC 11 Strength saving throw or be knocked prone. 

Clawed Stomp. Melee Weapon Attack: +8 to hit, reach 5 ft., one prone target. Hit: 9 (1d8 + 5) bludgeoning damage plus 3 (1d6) slashing damage. 

Tail. Melee Weapon Attack: +8 to hit, reach 20 ft., one target. Hit: 22 (5d6 + 5) bludgeoning damage plus 1 fire damage. 

Fireball. RangedWeapon Attack: +2 to hit, range 10/30 ft., up to three targets that are each within 5 feet of each other. Hit: 8 (2d8 - 1) fire damage.

Aggressive Roar. The charrog lets out a furious roar. Each creature within 10 feet of the Charrog must succeed on a DC 15 Wisdom saving throw or take 3 (1d6) thunder damage and have disadvantage on its next ability check, saving throw, or attack roll. If the Charrog is Enraged, the roar deals an additional 2 (1d4) fire damage.

Flame Jets (Recharge 4-6). The Charrog spews fire from its back and its front in 25-foot cones. Each creature in those areas must make a DC 16 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save or half as much damage on a success.

Fireball Belly-Flop (1/Short or Long Rest, Usable only when Enraged). The Charrog leaps into the air before crashing down, sending up a volley of blazing lava balls from all around it. Each creature within 40 feet of the Charrog when it crashes down must make a DC 17 Dexterity saving throw, taking 10 (3d6) bludgeoning damage + 45 (10d8) fire damage, be knocked prone, and takes 1 fire damage at the start of each of their turns for the next 1 minute on a failure, or half as much damage and no additional effects on a success. After using this attack, the ground within 30 feet of the Charrog becomes difficult terrain, and the Charrog stops being Enraged and has disadvantage on all of its attack rolls for the next 1 minute.