How does the Hydra work in Dungeon World?

Question

I'm looking at the description of the Hydra in Dungeon World (pg 248). It states that it has 16 HP and 2 Armor. So if it takes 16 points of damage it's dead. Seems straight forward.

However, It then says it can only be killed by a blow to the heart, and down the bottom though it talks about regeneration of body parts.

Not sure how to put these pieces together. Seems like unless they players actively state that they are attacking the heart, that the damage doesn't really matter.

Answer 1

Let's start on page 221, under Elements of a Monster. Under HP, it says HP is a measure of how much damage a monster can take before it dies, at which point it flatly dies. Just below that, it describes Special Qualities as "a guide to the fiction, and therefore the moves".

The crux of the issue with the Hydra is it has the Special Quality of "Only killed by a blow to the heart". As such, this overrides the more general rule about HP, and a hydra with 0 hp isn't, in fact, dead.

So, what happens?

You follow the fiction.

This hydra has taken enough stab wounds, magical assaults, and crushing blows to kill a dragon whelp, lich, or chimera. If a head was cut off and didn't regenerate yet, it's likely still gushing blood. It's still limping from that mace blow to the leg. That last magic missile to the heads probably just knocked it out, or it finally succumbed to the blood loss. Or maybe the Ranger got a lucky shot where the Wizard seared a hole in its chest earlier, and an arrow found its heart, killing it. But supposing it was only a flesh wound, the fiction should indicate the way it's out of the fight.

So, what happens next?

You follow the rules.

The Hydra lists "Regenerate a body part (especially a head)" as a Monster Move. Those are a specific kind of DM move. As such, that move is triggered at GM discretion (in lieu of any other DM move that could fit the fiction at the time), and when one of these three things happens:

• A golden opportunity (The rogue says "Do you guys think it's dead yet?", and it starts to quiver in response)
• A 6 or less on a player move (The ranger begins to search the body for the necessary healing potion reagents, and a new head forms and starts to sniff at him)
• Everyone looks to you to see what happens (nobody else is talking, and they're all looking at you like you're supposed to make a decision on something)

Or maybe none of those three things happen. Say the whole party decides they've vanquished the hydra, then wanders off. Pretty soon, the hydra will regenerate. If and when it does, the description requires that it come back stronger, and with more heads (if one was cut off in the battle). This is when the DM will follow the principles "make a move that follows", "think dangerous", "think offscreen, too", and "give every monster life". Those in turn help the DM fulfill the Agenda items "portray a fantastic world" and "fill the characters' lives with adventure".

Answer 2

You should go fiction first with the hydra

Tracking the exact number of hit points is more important for players' characters:

A character who is reduced to 0 HP immediately takes his Last Breath.

When we talk about a monster's hit points though, the precise number is not so relevant. Player do not know the hydra's HP, neither maximum nor current. Moreover, "a hydra is fully functional until you deal X damage to it; then it is dead" is the D&D interpretation of hit points. Damage works differently^* in DW:

When the harm is specific, like an orc pulling your arm from its socket, HP should be part of the effect but not the entirety of it

Likewise having your head chopped off is not HP damage, it’s just you being dead.

"A blow to the heart" is specific enough. You want to go fiction first, that's why you ask players to describe their actions — not just say "I attack the hydra", but say the details. Until they say "I strike the hydra to the heart" they can't kill it with conventional weapons, regardless of how many damage they deal in total. Body part regeneration is the in-world explanation of hydra's immortality. As the GM, you don't mention the hydra's hit points at all — instead, you say it is "unhurt", "wounded", "bleeding", "regenerating", "dead", whatever.

• If the hydra's HP is reduced to zero, but its heart wasn't injured, it regenerates its body parts; it might be incapacitated temporarily, up to the GM's discretion^**
• If the hydra gets a strike to its heart, it is probably^** dead
• If the hydra gets a killing blow to its heart, which reduces its HP to zero, it is definitely dead

See also A 16 HP Dragon

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^{* — "hit points" is a DW concept, inherited from D&D; it is normally absent in other PbtA games
** — encounter difficulty heavily depends on the GM's benevolence; it is normal for Dungeon World}