Chaos Sewer Hydra

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

Multiple Heads. The hydra has six heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened while more than one head is alive, 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 10 hit points for each head regrown in this way.

Chaotic Regeneration. 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 15 hit points for each head regrown in this way.

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

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

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

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 6) piercing damage, plus 7 (2d6) additional damage randomly for each bite:

D6   Damage Type
1     Acid
2     Cold
3     Fire
4     Thunder
5     Lightning
6     Poison

If a creature is hit by two or more different damage types in the same round, it must succeed on a DC 16 Constitution saving throw or be disoriented until the end of its next turn (disadvantage on attack rolls).

Thrashing Bulk (Recharge 5–6). The hydra violently lashes its necks and tail. Each creature within 15 feet must make a DC 17 Dexterity saving throw

• On a failure: 18 (4d8) bludgeoning damage and knocked prone
• On a success: Half damage, not knocked prone

The Chaos Hydra has 3 legendary actions, choosing from the options below. Only one option may be used at a time, and only at the end of another creature’s turn.

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

Tail Whip The hydra makes a tail attack against all creatures of its choice within 15 feet. Attack: +10 to hit Hit: 11 (1d10 + 6) bludgeoning damage 

Snap and Tear (Costs 2 Actions) The hydra makes one bite attack per two heads it currently has (rounded down). 

Instant Regeneration (Costs 3 Actions) If the hydra has three or fewer heads, it immediately regrows two heads for each head lost since its last turn and regains 15 HP per head regrown.