Brachydios

Multiattack: The Brachydios makes 3 attacks, one with its Head Slam and two with its Punch. 

Punch: Melee weapon attack, +10 to hit, reach 5ft, one target. Hit 29(4d10+7) bludgeoning damage plus 7(2d6) fire damage.

Head Slam: Melee weapon attack, +10 to hit, reach 10ft, one target. Hit 33(4d12+7) bludgeoning damage plus 7(2d6) bludgeoning. 

Chain Reaction(Recharge 6): The Brachydios slams it's head spike into the ground, causing a series of eruptions and explosions in front of it. All creatures in a 20ft cone must make a DC 19 Dexterity saving throw, taking 105(30d6) fire damage on a failed save, and half as much on a successful one. 

The Brachydios can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Brachydios regains spent legendary actions at the start of its turn. 

Detect: The Brachydios makes a Wisdom(Perception) check. 

Slime Explosion: The Brachydios makes a Chain Reaction attack, but only if it is available. 

Explosive Charge(Cost 2 Actions): The Brachydios moves up to its speed. During this movement, it can move through the space of other creatures and ignores difficult terrain. Any creature the Brachydios moves through must succeed on a DC 19 Dexterity saving throw or take 14(2d6+7) fire damage and be knocked prone.