By Alan McCoy from Dungeons & Dragons: Fundamentals
How to build challenging
encounters against a Hydra
Note: This has been prepared utilizing only the 5E Core Rules. The Hydra can be found in the Monster Manual page 190. It is classified as a Huge Monstrosity, which means that it occupies a 15-foot square of space.
The hydra is a huge reptilian horror, with a crocodilian body, and multiple heads on long, serpentine necks. It is known that the heads can be individually attacked and severed, but that they magically grow back.
Legend has it that the hydra race is a form of dragon-kin, an offshoot of the race of dragons that was accidently formed when Tiamat, the Goddess of Evil Dragon kind slew a rival named Lernaea. The hydras formed wherever one of Lernaea’s drops of blood fell across the multiverse.
Whatever their origin, these creatures are gluttons, and are constantly hungry. They are nomadic, eating their way through an area, destroying anyone that comes to challenge it, then moving on to another hunting ground. Their hunger is legendary, and in lean times they have even been known to snatch off one of their own heads to devour it.
Unlike Dragons, hydrae cannot fly, but they are powerful swimmers.
Hydra’s rarely take a lair, they do not have the need, and there is rarely enough food any given area to support their appetite long enough to allow them to settle and procure a lair. They seem to have no trouble from the elements, so they really don’t need a lair in any case. Hydra rarely stop to rest or to sleep, usually one or more of the heads will sleep in shifts while the others control the body.
There are rumors of Hydra’s that have breath weapons, and that regenerate not just single heads, but multiple heads when a head is severed.
Most hydrae have five to seven heads, sightings of a creature with as few as three heads have been documented as have sightings of creatures with as many as thirteen.
DM note: The multiple head regeneration is true; the breath weapon is not normally true. If you choose to make it true, it is recommended that you do not utilize a fire or acid breath weapon. As you know from the Troll, both substances interfere with regeneration.
Step 1) Let’s Review what we know about the Hydra:
Their strength and constitution is exceptional, and their dexterity is above average. Their wisdom is average, their charisma below average and they are exceptionally stupid.
Hydra have a movement speed of 30-feet per round on land and while swimming.
Hydrae can hold their breath for an hour or more. They can breathe with a single head if necessary.
If a Hydra has more than one head available, it has ADVANTAGE on saving throws against being blinded, charmed, deafened, frightened, stunned or knocked unconscious.
Damage to an individual head does not harm the hydra as a whole, if the for every 25 HP of damage done to a hydra a head is considered severed, and will be magically replaced by two more heads (that show up at the end of its turn) unless the damage done to the hydra was caused by fire. Regenerating heads restore 10 hp each to the HP total of the Hydra.
The serpentine necks of each of the hydra’s heads are greater than 10-feet long and have an attack reach that can attack anyone within ten feet of the hydra’s main body. Further, each head can attack independently and has a separate reaction as well.
Not RAW – If more than two heads attack the same target, they begin a vicious tug of war to attempt to pull the target away and eat it. Suggested Mechanics in the Tips and Observations section.
Step 2) Determine the probable Strategy.
They hydra is a true monster, with nothing but a voracious appetite that drives its behavior. It does have a normal instinct for self-preservation, but this has been blunted by the fact that its heads grow back whenever they are severed.
The hydra will likely charge the closest foe and attempt to destroy him or her first. Anyone armed with fire or acid will cause the Hydra to target them as primary opponents.
Step 3) Determine Tactics
While the hydra itself is quite stupid, it does have few programed instincts, the first is to attack anyone wielding fire first, the second is that armored creatures are hard to eat, and their third is that biting one creature until they are dead is better than biting lots of creatures stay alive and try to fight you.
By sacrificing one of its bite attacks the Hydra can make a SHOVE ATTACK against one of its foes, knocking them down and trampling them. This requires that the target be within movement range of the foe, and will toss them back, to land 15 feet away and prone. As the hydra is at least two sizes larger than the target, it has ADVANTAGE on its Strength check.
Note: While the hydra is stupid, it is not so stupid that it will have more than two heads occupied in a ‘tug of war’ over a meal at the same time.
Step 4) DM Tips and Observations
The Swim Speed of the Hydra should be increased to 60-feet. Let the water be their primary method of transportation.
The hydra’s heads should be given the bonus action disengage, to reflect the lightning strikes it makes with its heads, making it immune to attacks of opportunity.
If you choose to use the SHOVE attack, in addition to the push, the attack should be given damage equal to 1d10+5 (normal bite attack) and they must take at DC 10 or DC damage taken from the blow (whichever is higher) or be Stunned until their turn in the next round.
Head Tug of War – Each head makes a grapple check vs the target, if they both grab hold, the target is RESTRAINED and the ‘TUG of WAR’ commences. Each head does automatic damage to the target each round. When the targets total HP drops to zero, they are torn apart and die instantly. The target is allowed Athletics and Dexterity checks at disadvantage to escape from one of its captor heads each round.
If you do choose to provide a breath weapon attack for these creatures, it is suggested that you utilize the Paladin SMITE as your guide, allowing each HEAD to do 2d8 additional damage once each day. REGENERATED HEADS count as NEW heads for the purposes of their daily smite.