Behidra

Hold Breath. The Behidra can hold its breath for 8 hours.

Legendary Resistance(3/day). If the Behidra fails a saving throw, it can choose to succeed instead. 

Multiple Heads. The Behidra has seven heads. Whenever the Behidra takes 50 or more damage on a single turn, one of its heads dies. If all its heads die, the Behidra dies. 

At the end of each of its turns, it grows two heads for each of its heads that died since its last turn. The Behidra regains 20 hit points for each head regrown in this way. If the Behidra takes acid damage, it must make a DC 25 Constitution saving throw. On a failure, it doesn't regrow any heads on its next turn. 

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

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

Multiattack: The Behidra can use Staggering Shockwave. It then makes as many Bite attacks as it has heads. 

Bite: Melee weapon attack, +16 to hit, reach 15ft, one target. Hit 13(1d10+8) piercing damage plus 5(1d10) lightning damage. If this attack lands a critical hit or exceeds the targets AC by 10 or more, the target is grappled(escape DC 18). Until this grapple ends, the target is Restrained, and the Behidra makes one Bite attack against it per turn. The Behidra has seven heads, each of which can grapple a target.

 Lightning Breath(Recharge 5-6): The Behidra exhales lightning in a 60ft line that is 10ft wide. Each creature in that line must make a DC 23 Dexterity saving throw. On a failure, the target takes 22(4d10) lightning damage plus an additional 11(2d10) lightning damage for each head the Behidra has beyond one. On a success, the target takes half as much damage. Creatures that are grappled by the Behidra automatically fail this saving throw. 

Staggering Shockwave: The Behidra unleashes a debilitating surge of electrical and thunderous energy into the air surrounding it. Each creature within 60ft of the Behidra of its choice must make a DC 23 Constitution saving throw. On a failure, until the end of the target's next turn, its speed becomes 0 and it can't take reactions. Additionally, the target can either take an action or bonus action on its turn, not both, and regardless of its abilities or magic items, it can't make more than one melee or ranged attack during its turn.

The Behidra 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 Behidra regains spent legendary actions at the start of its turn. 

Move: The Behidra moves up to its speed. 

Reactive Stance: If the Behidra takes damage from a creature within 15ft of it before the start of its next turn, it can use the reaction granted by its Reactive Heads trait to make three Bite attacks against that creature, regardless of how many heads it has. A target can't become grappled by these Bite attacks.

Swallow(Cost 2 Actions): The Behidra makes one Bite attack against a large or smaller creature it is grappling. If the attack hits, the target is swallowed, and the grapple ends. A swallowed creature is Blinded and Restrained, has total cover against attacks and other effects outside the Behidra, and takes 28(8d6) acid damage plus 28(8d6) lightning damage at the start of each of the Behidra's turns. It also automatically fails the saving throw against the Behidra's Lightning Breath and takes maximum damage. It also can still be affected by the Behidra's Staggering Shockwave. 

If the Behidra takes 50 or more damage on a single turn from a swallowed creature, it must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall Prone in a space within 10ft of the Behidra. This damage doesn't cause one of the Behidra's heads to die. If the Behidra dies, a swallowed creature is no longer Restrained and can escape from the corpse using 30ft of movement, exiting Prone. 

Snapping Frenzy(Cost 3 Actions): The Behidra moves up to twice its speed, making one Bite attack against each target of its choice that is within 15ft of it at any point during this movement. It has advantage on each of these attack rolls. A target can't become grappled by any of these attacks.