Alchemical Hydra

• Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.
• Multiple Heads. The hydra has five heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.
  Whenever the hydra takes 30 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.
  At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The hydra regains 12 hit points for each head regrown in this way.
• Wakeful. While the hydra sleeps, at least one of its heads is awake.

Multiattack. The dragon can use its Frightful Presence. It then makes as many bite attacks as it has heads.

• Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: (2d10 + 7) piercing

• Frightful Presence. Each creature of the hydra's choice that is within 120 ft. of the dragon and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

• Lightning Breath (Recharge 5-6). The hydra exhales lightning in a 90-foot line that is 5 ft. wide from each of its heads. Each creature in that line must make a DC 19 Dexterity saving throw, taking 33 (6d10) lightning damage on a failed save, or half as much damage on a successful one. 

The Alchemical Hydra can take 3 legendary actions, 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. Adult Blue Dragon regains spent legendary actions at the start of their turn.

• Detect.The Hydra makes a Wisdom (Perception) check.
• Snaking Maw. The Hydra attempts to wrap around a creature within range (10 feet) with its neck, they must make a contested Dex (acrobatics) or Str (athletics) check or be grappled by the Hydra
• Searing Ooze. The hydra Releases a gelatinous Ooze from the the separations in its scales, creatures in contact must make a DC 20 dex save taking 14 (2d6+7) acid damage on a failure, or half on a success. A creature Grappled by the Hydra automatically fails