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In a session where I was the DM, I made my players fight a hydra. In the end I had to end the battle by stating the hydra became unconscious. It was at 0 hitpoints for a good 4 turns before I made my decision.

I took so long to make my decision because of a statement made in the hydra's information in the monster manual pg. 190

Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.

So at first I assumed until all the hydra heads fall off it would not die. But my players were unable to kill the heads resulting in the hydra just falling to 0 health without any heads having been killed. If I let them try and cut off all the heads they would never had done it.

In short my question is What are the different ways to kill a hydra

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The rule about all of its heads dying is an exception particular to the hydra. It is not immune to the effects of being defeated when reduced to 0 hp.

If the hydra were immune to defeat at 0 hp, its description would have to say so.

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    \$\begingroup\$ I guess the rule is there not as defensive armour (you must destroy all heads to defeat this creature) but for the equally odd situation of continuing to fighting against the headless hydra (you may destroy all the heads but then there is no point in the body keeping going uselessly). \$\endgroup\$ – Neil Slater May 12 '16 at 9:23
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    \$\begingroup\$ So there's a reading of the entry that could take it as needing to kill all heads with fire to kill it. "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 fire damage since its last turn. The hydra regains 10 hit points for each head regrown in this way. " This could be taken to mean that even after it's died all heads have died and now grow two new heads. I don't think it's supposed to be read that way but it could be interpreted that way. \$\endgroup\$ – Ringo_St R May 12 '16 at 12:30
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    \$\begingroup\$ That reading would require the hydra's special traits to be active even after it dies, and it would be the only creature in the game that does this without specific mention of the fact. \$\endgroup\$ – Polisurgist May 12 '16 at 15:49
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    \$\begingroup\$ @Polisurgist Remember though that nothing dies at 0hp; treating monsters that hit 0hp as “might as well be dead” is a DMing shortcut of convenience only and doesn't override the actual rule that they need to fail three death saves or take massive damage before properly dying. \$\endgroup\$ – SevenSidedDie May 13 '16 at 0:03
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    \$\begingroup\$ Generally they do. MM p. 7: "A monster usually dies or is destroyed when it reaches 0hp. For more on hit points, see the Players Handbook." PHB p. 198: "Most DMs have a monster die the instant it drops to 0 hit points...Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters." So, "most," with particularly special villains marked as an exception. \$\endgroup\$ – Polisurgist May 13 '16 at 15:34

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