Blooming Hydra

Legendary Resistance (3/Day). If the hydra fails a saving throw. It can choose to succeed instead.

Acidic Blood. When the hydra takes piercing or slashing damage, each creature within 5 feet of the hydra takes 9 (2d8) acid damage.

Hold Breath. The hydra can hold its breath for 1 hour.

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, or knocked unconscious. Whenever the hydra takes 25 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 10 hit points for each head regrown in this way.

Rip it up. If two or more bite attacks are successful on a single target, the hydra may grapple the player character. The player may attempt to escape with a strength saving throw or be grappled and take (2d6) slashing damage as the hydra tries to rip them in two. If the hydra takes 25+ damage before its next turn the player will be released as one of the heads is destroyed. If not, they can make a strength saving throw and if failed is delivered enormous damage and reduced to 0 hit points and forced to make a Death saving throw.

Wakeful. While the hydra sleeps, at least one of its heads is awake.

The Last stand. When the hydra reaches 0 hit points it makes one final Multiattack against every player character within 10 ft. before finally succumbs to its wounds.

Multiattack. The hydra makes as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 49 (8d10 + 5) piercing damage.

The hydra may use one of the following abilities as an action/bonus action. 

Attack Pattern: Roar of the Dead -> Costume Change -> Multiattack -> Neck Smash -> Three heads are better than two -> Tail Lash -> Drag and drop.

Roar of the dead. Once per encounter the hydra funnels air into its lungs and makes the air and earth shake with force and ferocity, Each player must succeed on a wisdom saving throw or become frightened until the start of their next turn.

Costume Change. +2 to its AC until the start of its next turn, as its hide shifts and changes both in colour and texture making its flesh thick and scaly.

Neck Smash. Each player character within 10ft. of the hydra makes a strength saving throw or be knocked prone. As it makes a violent sweep with its long serpentine necks.

Three heads are better than two.  At the end of its turn the hydra regrows a additional head, as it ingests a huge swirl of corrupted energy.

Tail Lash. Targeted player must succeed on a Dexterity saving throw or take (2d8) bludgeoning damage. The tail wraps itself around the player characters body and holds them in place, restraining them until they wrestle free (strength saving throw)

Drag and drop. The hydra makes a bite attack +8 to hit, reach 10 ft., one target. Hit: 49 (8d10 + 5) piercing damage. If successful  the player characters makes a DC Strength saving throw to wrestle free, should they fail, the hydra lifts the player 10ft in the air and smashes the player into the ground suffering (2d6) bludgeoning damage.

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

The  hydra can take three 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. Spent Legendary Actions are regained at the start of each turn.

Costume Change. The hydra uses the Costume Change Hydra Ability

Multiattack. (costs two Legendary Actions) The hydra makes a Multiattack

Neck Smash. (costs two Legendary Actions) The hydra uses the Neck Smash Hydra Ability