Cave Hydra

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

Multiple Heads. The hydra has three 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.

Regeneration: At the start of the cave hydra's turn, regain 20 hit points. When the cave hydra loses one of it's three heads, randomly choose which of it's multiple heads is removed. If the cave hydra lost a head after taking more then 20 points of damage while one head remains, if the cave hydra takes a long rest, they regain all lost heads. 

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

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

Spider Climb. The cave hydra can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. The cave hydra makes each of it's various bite or spit attacks that is available. If a head of the unique bite attack is removed, the cave hydra can't use their bite or spit attack. 

Poison Bite. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 10 (1d10 + 5) piercing damage plus 10 (3d6) poison damage.

Acid Bite. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 10 (1d10 + 5) piercing damage plus 10 (3d6) acid damage.

Death Bite. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 10 (1d10 + 5) piercing damage plus 10 (3d6) necrotic damage.

Spit Poison. Ranged Weapon Attack: +6 to hit, range 30/45 ft., one creature. Hit: The target must make a DC 15 Constitution saving throw, taking 18 (8d6) poison damage and m [condition]poisoned[/condiition]  for 1 minute on a failed save, or half as much damage on a successful one. 

Spit Acid. Ranged Weapon Attack: +6 to hit, range 30/45 ft., one creature. Hit: The target must make a DC 15 Constitution saving throw, taking 18 (8d6) acid damage and non-magical armor lose 1 AC permanently and non-magical weapons gain -1 to hit  on a failed save, or half as much damage on a successful one.

Spit Death. Ranged Weapon Attack: +6 to hit, range 30/45 ft., one creature. Hit: The target must make a DC 15 Constitution saving throw, taking 18 (8d6) necrotic damage and gain a level of exhaustion on a failed save, or half as much damage on a successful one.