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