Alzera the Red, Firestorm Hydra

Amphibious. The dragon can breathe air and lava.

Legendary Resistance (4/Day). If the dragon fails a saving throw, it can choose to succeed instead. If this resistance is used against a spell then the spell slot for that spell isn't used. 

Flame Aura. Each creature that starts its turn within 15ft of Alzera takes 5 fire damage due to the extreme heat emitted by the dragon's skin. 

Presence of the inferno. The first time a creature starts their turn while in combat with Alzera and attempts to use their action, they must make a Wisdom saving throw. DC-19. On a fail, they become feared until the start of their next turn. When feared in this way creatures have disadvantage on all attack rolls and saving throws. Furthermore, they cannot use their movement speed.

Weakspot. Alzera has a second AC equal to its current AC + 9 (28). If a creature attacking Alzera with an attack roll hits this second AC and describes its attack as hitting Alzera's weak spot in its chest, treat the attack as a critical hit. 

Missing Scales. Alzera has a third AC equal to its current Ac - 4 (15). Creatures that hit this lower AC but not the normal AC of 19 and describe the attack as hitting a missing scale the attack still hits Alzera but only deals half damage. Any effects that apply on the hit don't take effect. (such as Hunters Mark). Furthermore when Alzera's Hp drops below 50 his immunities become resistances. 

Multiattack. Alzera makes three attacks: one with its bite and two with its off-head bites. Alzera can substitute one of its off-bite attacks with a Claw Attack. 

Bite. Melee Weapon Attack:  +11 to hit. reach 10ft, one target,  17 (3d6 +6) piercing damage, and 11 (3d6) fire damage. Total = 28. If a creature is hit it must then make a strength saving throw. DC - 19. On a fail, they are thrown 60ft. 

Off Bite. Melee Weapon Attack:  +11 to hit. reach 10ft, one target, 19 (5d4 +6) piercing damage, and 3 (1d4) fire damage. Total = 22

Claw. Melee Weapon Attack:  +11 to hit. reach 10ft, one target, 11 (2d4 +6) slashing damage. Creatures hit by this attack are pushed 15ft away from Alzera. Total = 11

Tail Attack. Melee Weapon Attack:  30ft cone. Creatures must make a Strength saving throw. DC-19. On fail, they take 20 (4d6 + 6) bludgeoning damage, are knocked prone, and concussed for 1 turn. On a concussed creature's turn, it must choose whether it uses an action, a bonus action, or its movement speed. On a success, they are not concussed, aren't knocked prone, and take half damage. 

Triple Fire Breath streams.  Alzera Blasts out 1 to 3 fire breath streams from its heads each affecting a 60ft long and 15ft wide Line originating from Alzera. They can stack over each other. Each creature in a breath stream must make a dexterity saving throw DC-19. On success, they take half the damage. Each breath stream deals 42 (12d6) fire damage.

If a creature is hit by more than one breath they only make one dexterity save but take the damage of multiple breaths. If a creature is hit by more than one breath it also must make a Constitution saving throw. Dc-19. On fail, they are burned and will take a 3d10 fire damage at the start of their next turn. Alzera can blast 9 fire breath streams per Long rest.

Draconic Smell. As a bonus action, Alzera can hone his sense of smell. Alzera will immediately know the exact location of all creatures within 240ft unless a creature chooses to use its reaction to roll stealth against Alzera's passive perception.

Claw Pound. As a bonus action, Alzera can slam its claw into the ground causing one rock to fall from the ceiling as described in its lair actions. 

Malleable Hide. As a reaction to taking an instance of damage, Alzera can reduce the instance of damage by 18 (4d8). Using this feature lowers Alzera's AC by 1 but no more than 10 at which point Alzera can't use this feature until the next long rest when its AC returns to normal. 

Alzera knows 2 legendary actions per turn, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Each Action can only be used once per turn. 

Roaring Fireball. 1 action: Alzera shoots out a great fireball from its mouth. Each creature in a 20-foot-radius sphere centered on a point within 60ft must make a Dexterity saving throw. DC - 19. A target takes 28 (8d6) fire damage on a failed save, or half as much damage on a successful one.

Tail Attack. 1 action: make one Tail attack. 

Last Ditch. Immediately after Alzera is reduced to 0 hp It will stand back up with 1 hp. Alzera then makes 1 fire breath stream attack. This Fire breath is treated as if 2 breath attacks are stacked over each other.