This is a legendary hydra I plan on having my players encounter in our next session. Thought I'd share it in case anyone found it interesting. It will be at the bottom of a 30' deep body of water, with the PC's on platforms and walkways that are 10' above the surface of the water.
Prismatic Hydra
Type: Monstrosity
Size: Gargantuan
Speed: 30 ft., swim 30 ft.
AC: 15 (Natural Armor)
HP: 230 (20d12+100)
Alignment: Chaotic Neutral
Challenge Rating:11
Skills: Perception +6
Passive Perception: 16
Senses: Darkvision 60 Ft.
Traits
Feat: Warcaster (allows it to use a beam attack as an opportunity attack)
Legendary Resistance (3/Day):If the hydra fails a saving throw, it can choose to succeed instead.
Immunities: Disease
Resistances: Each head is resistant to the type of damage it can do.
Hold Breath: The hydra can hold its breath for 1 hour.
Multiple Heads:The hydra has five heads. (Choose or roll on the Hydra Head chart to see which colors it starts with, 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 25 or more damage in a single turn, one of its heads dies. If all its heads die before its turn, the prismatic hydra will blindly flee (usually underwater) and attempt to retreat into an underwater burrow and hibernate until it can regrow its heads.
At the end of its turn, it grows two heads for each of its heads that died since its last turn. Roll on the Hydra Head chart for each head to see what type of head regrows. The hydra regains 10 hit points for each head regrown in this way. (All extra heads are reabsorbed into the hydra’s body at the end of a long rest.)
Independant Heads: The heads can be targeted individually, and all take damage individually if they are caught in the same AoE effect. They are considered Large creatures. The heads all share the same HP pool. The Prismatic Hydra prefers to stays submerged at least 10’ below the water during combat, but as each of its heads has a 30’ long neck and a 60’ range eyebeam, this is usually not a hindrance. The heads are somewhat chaotic in nature and do not usually exhibit teamwork or focus their attacks unless there is only one target.
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.
Prismatic Heads: Each head can shoot a beam of energy from its mouth instead of making a bite attack. Dexterity Save DC: 17, full effect on a failed save, no effect on a successful save. Range: 60’
Hydra Head Chart:
Digestion Dimension: a pocket dimension that can store up to 16 medium creatures or 4 large creatures or 1 huge creature. The walls of the digestion dimension feel like living flesh coated in digestive juices. Creatures in the digestion dimension take 5d6 acid damage (con save for half) at the beginning of each of their turns. Dealing 25 points of damage in one turn to the walls of the digestion dimension causes a rupture that will expel one creature out of the dimension via the violet hydra’s mouth. Dealing 50 points of damage to the walls of the digestion dimension will collapse the dimensions, destroying the violet hydra head and expelling all creatures via the neck stump. Likewise, dealing 25 point of damage the violet hydra head from outside will severe the head, collapse the dimension, and expel the contents of the dimension via the neck stump.
Actions
Multiattack: The hydra makes as many bite attacks as it has heads. It may replace up to 3 bite attacks with beam attacks on different targets (only targeting the same creature if no other targets are available) - the heads work independently and don’t use teamwork
Bite: Melee Weapon Attack: +9 to hit, reach 30 ft. (effectively 20 ft as the main body stays submerged 10’ below water), one target.
Hit: 10 (1d10 + 5) piercing damage.
Legendary Actions
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. Spent legendary actions are regained at the start of each turn.
Beam Attack: One head that has not made a beam attack since the beginning of the hydra’s last turn can make a beam attack against a random enemy.
Bite Attack:May make a bite attack against a random enemy within range.
Regrow Head (costs 2 actions):If the hydra currently has less than 5 heads, it may immediately regrow a head and heal 10 hit points.
Prismatic Hydra
Type: Monstrosity
Size: Gargantuan
Speed: 30 ft., swim 30 ft.
AC: 15 (Natural Armor)
HP: 230 (20d12+100)
Alignment: Chaotic Neutral
Challenge Rating:11
| STR | DEX | CON | INT | WIS | CHA |
| 20 (+5) | 20 (+5) | 20 (+5) | 2 (-4) | 10 | 7 (-2) |
Skills: Perception +6
Passive Perception: 16
Senses: Darkvision 60 Ft.
Traits
Feat: Warcaster (allows it to use a beam attack as an opportunity attack)
Legendary Resistance (3/Day):If the hydra fails a saving throw, it can choose to succeed instead.
Immunities: Disease
Resistances: Each head is resistant to the type of damage it can do.
Hold Breath: The hydra can hold its breath for 1 hour.
Multiple Heads:The hydra has five heads. (Choose or roll on the Hydra Head chart to see which colors it starts with, 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 25 or more damage in a single turn, one of its heads dies. If all its heads die before its turn, the prismatic hydra will blindly flee (usually underwater) and attempt to retreat into an underwater burrow and hibernate until it can regrow its heads.
At the end of its turn, it grows two heads for each of its heads that died since its last turn. Roll on the Hydra Head chart for each head to see what type of head regrows. The hydra regains 10 hit points for each head regrown in this way. (All extra heads are reabsorbed into the hydra’s body at the end of a long rest.)
Independant Heads: The heads can be targeted individually, and all take damage individually if they are caught in the same AoE effect. They are considered Large creatures. The heads all share the same HP pool. The Prismatic Hydra prefers to stays submerged at least 10’ below the water during combat, but as each of its heads has a 30’ long neck and a 60’ range eyebeam, this is usually not a hindrance. The heads are somewhat chaotic in nature and do not usually exhibit teamwork or focus their attacks unless there is only one target.
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.
Prismatic Heads: Each head can shoot a beam of energy from its mouth instead of making a bite attack. Dexterity Save DC: 17, full effect on a failed save, no effect on a successful save. Range: 60’
Hydra Head Chart:
| Head Color | Damage Resistance | Prevents Head Regrowth | Effect on a Failed Save |
| 1) Red | Fire | Cold damage | 10d6 fire damage |
| 2) Orange | Acid | Lightning damage | 10d6 acid damage |
| 3) Yellow | Lightning | Acid damage | 10d6 lightning damage |
| 4) Green | Poison | Any effect that fixes he poisoned condition | 10d6 poison damage |
| 5) Blue | Cold | Fire damage | 10d6 cold damage |
| 6) Indigo | Slashing/ Piercing | Thunder damage | Restrained. New Con save at end of turn: 3 failures = Petrified/3 successes ends effect |
| 7) Violet | Force | Necrotic damage | Blinded. New Wis save at beginning of turn. Failure = transported to Digestion Dimension. (See below) |
| 8) Black | Necrotic | Radiant damage | 10d6 necrotic damage |
| 9) Gray | Necrotic | Radiant damage | Animate Dead (Can animate severed hydra heads as undead giant constrictor snakes, size large instead of huge) |
| 10) White | Radiant | Necrotic damgae | 10d6 radiant damage |
Actions
Multiattack: The hydra makes as many bite attacks as it has heads. It may replace up to 3 bite attacks with beam attacks on different targets (only targeting the same creature if no other targets are available) - the heads work independently and don’t use teamwork
Bite: Melee Weapon Attack: +9 to hit, reach 30 ft. (effectively 20 ft as the main body stays submerged 10’ below water), one target.
Hit: 10 (1d10 + 5) piercing damage.
Legendary Actions
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. Spent legendary actions are regained at the start of each turn.
Beam Attack: One head that has not made a beam attack since the beginning of the hydra’s last turn can make a beam attack against a random enemy.
Bite Attack:May make a bite attack against a random enemy within range.
Regrow Head (costs 2 actions):If the hydra currently has less than 5 heads, it may immediately regrow a head and heal 10 hit points.
