Hydra
Huge monstrosity · CR 8 · 3,900 XP
15 AC · 184 HP
Roll the Hydra
Press a roll button. The dice land here.
The Hydra punishes the obvious plan, then rewards the smart one
How to run it. 184 Hit Points, AC 15, Speed 40 feet and Swim 40 feet. It bites once per head, +8 for 1d10 + 5, with 10-foot reach. It starts with five heads. Against AC 16 that is about 34 damage a round; at six heads 41; at seven 48.
The rule that matters. Multiple Heads. Any turn it takes 25 or more damage, one head dies. At the end of its own turn it grows two heads for every head lost since its last turn, and regains 20 Hit Points, unless it took Fire damage since its last turn. Without fire, every head you cut nets it one more head and 20 more Hit Points. With fire, every cut is a real loss.
Getting to 25. One fighter landing two greatsword hits (2d6 + 3 each) reaches 25 only about 10% of the time. A DC 15 Fireball (8d6) does it in one go 76% of the time if the hydra fails its save, and its Dex save of +1 fails 65% of the time. So one Fireball kills a head about half the time and stops regrowth whatever it rolls, because any Fire damage counts.
Other rules. Every head past the first gives it an extra Reaction, only for Opportunity Attacks. Walking past a five-headed hydra costs you up to four free bites. It cannot be Blinded, Charmed, Deafened, Frightened, Stunned or knocked Unconscious, and it can hold its breath for an hour, so a lake is its home ground.
The common mistake is not telling the players what happens. Describe two heads bursting out of the stump. Describe the blackened stump that stays closed after the torch. They will work out the rest.
For players. Fire first, then cut. Save the big hits for the turn after someone lands Fire damage.
The human angle. In 1976 Barry Staw had business students decide where to invest; those personally responsible for a losing choice put more money into it, not less. He called it escalating commitment. A table that keeps cutting heads while the hydra grows is doing the same thing with swords.
Source: Staw, B. M. (1976). Knee-deep in the big muddy: a study of escalating commitment to a chosen course of action. Organizational Behavior and Human Performance, 16(1), 27–44. doi:10.1016/0030-5073(76)90005-2
Stat block
- Type
- Huge monstrosity, unaligned
- Armor Class
- 15
- Hit Points
- 184 (16d12 + 80)
- Speed
- 40 ft., swim 40 ft.
- Challenge
- 8 (3,900 XP), proficiency +3
- Condition immunities
- Blinded, Charmed, Deafened, Frightened, Stunned, Unconscious
- Senses
- darkvision 60 ft., passive perception 16
- Languages
- None
| STR | DEX | CON | INT | WIS | CHA |
|---|---|---|---|---|---|
| 20 | 12 | 20 | 2 | 10 | 7 |
Hydra odds against your Armor Class
Chance to hit, then average damage per attack once misses and critical hits are counted.
| Attack | AC 12 | AC 14 | AC 15 | AC 16 | AC 18 | AC 20 |
|---|---|---|---|---|---|---|
| Bite (+8) | 85% 9.2 dmg | 75% 8.2 dmg | 70% 7.6 dmg | 65% 7.1 dmg | 55% 6.1 dmg | 45% 5 dmg |
| Full turn (1 attacks) | 9.2 dmg | 8.2 dmg | 7.6 dmg | 7.1 dmg | 6.1 dmg | 5 dmg |
Hydra hit points
The stat block uses the average, 184. Rolled, 16d12 + 80 lands anywhere from 96 to 272.
| Hit points | Chance |
|---|---|
| 140 or more | >99% |
| 184 or more | 51% |
| 228 or more | <1% |
| 254 or more | <1% |
Traits
Hold Breath. The hydra can hold its breath for 1 hour.
Multiple Heads. The hydra has five heads. Whenever the hydra takes 25 damage or more on a single turn, one of its heads dies. The hydra dies if all its heads are dead. At the end of each of its turns when it has at least one living head, the hydra 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 20 Hit Points when it grows new heads.
Reactive Heads. For each head the hydra has beyond one, it gets an extra Reaction that can be used only for Opportunity Attacks.
Actions
Multiattack. The hydra makes as many Bite attacks as it has heads.
Bite. Melee Attack Roll: +8, reach 10 ft. Hit: 10 () Piercing damage.
Other CR 8 monsters
Assassin 
Chain Devil 
Cloaker 
Frost Giant 
Hezrou 
Spirit Naga 
Tyrannosaurus Rex 
Young Bronze Dragon 
Young Green Dragon
This page includes material from the System Reference Document 5.2.1 (“SRD 5.2.1”) by Wizards of the Coast LLC, available at dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License. The averages, odds and tables are ours.