It would be ridiculous to try and out-do what Sean K. Reynolds has already brilliantly done for D&D 3.5 in The New Argonauts so at the start of every one of these posts I’m going to 1) thank him for making that supplement (thank you Sean!) and 2) tell you to go download it (it’s free–give it 5 stars because damn). This blog series is not intended to be a reproduction of that product, only a conversion update. Go get it.

Post 1: What You Need To Play

Master Page for D&D 5E Ancient Greece/Monster List

Greater Hydra

Huge monstrosity, unaligned
Armor Class 16 (natural armor)
Hit Points 207 (18d12+90)
Speed 30 ft., swim 30 ft.

Skills Perception +13
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 120 ft., passive Perception 23
Challenge 12 (8,400 XP)

Hold Breath. The greater hydra can hold its breath for 1 hour.
Immortal Head. One of the greater hydra’s heads (usually near the center) cannot be removed until all of the other heads are dead.
Multiple Heads. The greater hydra has nine heads. While it has more than one head, the greater hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. Whenever the greater hydra takes 30 or more damage in a single turn, one of its heads dies. If all its heads die, the greater 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 acid or fire damage since its last turn. The greater hydra regains 10 hit points for each head regrown in this way.
Reactive Heads. For each head the greater hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.
Wakeful. While the greater hydra sleeps, at least one of its heads is awake.

ACTIONS
Multiattack. The greater hydra makes as many bite attacks as it has heads.
Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 (1d10+5) piercing damage.
Breath Weapon (Recharge 6). The greater hydra exhales deadly poison in a 50-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 44 (8d10) poison damage on a failed save, or half as much damage on a successful one.

The original Greek hydra was a unique female Greek monster born of Echidna and Typhon, defeated by Heracles as one of his twelve labors. It is recreated here as the greater hydra with all nine heads and a breath so poisonous that it can kill weak creatures that come close to it. One of its heads was immortal and could not be killed, so Heracles buried the severed head under a rock. The Greek hydra’s blood was also deadly poison and weapons dipped in its blood could kill with a scratch (see the Magic chapter in The New Argonauts). The stories disagree on how many heads the hydra had (many say 9 but some say 50), so consider removing the nine-head limit to create a truly monstrous and terrifying hydra (though it’s reasonable only a limited number of its heads should be able to attack any particular creature).

Though not good at hiding, hydras are smart enough to wait in pools of water in their natural swampy habitat, concealing most of their body and appearing to be nothing more than a large snake (they usually array their heads in different directions so they can look all around without appearing to be a snake den). They charge at any creature big enough to eat and may focus their attacks on one creature or split them up if they’re taking damage from multiple enemies.

An Intelligence (History) check reveals the following information: 8—The hydra is a swampdwelling beast with many heads, and each time you cut off a head it grows another two in its place. 13—The only way to keep a head from regrowing is to sear the neck with fire or acid. A hydra’s breath is poisonous to anyone who comes near it. 20—One of the hydra’s heads is immortal and its body cannot be killed while that head remains in place.